Mustard Algae vs Green Algae: How to Tell and How to Kill It

If your pool has a color other than blue, you've got algae. But not all algae is the same — and treating the wrong type the wrong way will waste your time and money. In Texas, we mainly deal with two types: green algae (the most common) and mustard/yellow algae (the most stubborn). Here's how to identify which one you have and exactly how to get rid of it.

Quick Identification

Green Algae: The Common Enemy

Green algae is what most pool owners think of when they hear "algae." It turns your water green, coats the walls and floor with a slimy film, and multiplies fast in warm water. In Texas, green algae can go from "barely visible" to "swamp" in 48 hours during summer.

How to Kill Green Algae

1. Test and balance pH — Get pH to 7.2-7.5 first. Chlorine is much less effective at high pH.
2. Brush the entire pool — Walls, floor, steps, behind ladders, around lights. Break the biofilm.
3. SLAM the pool — Shock Level and Maintain. Raise FC to shock level based on your CYA and keep it there. See our SLAM guide for the full process.
4. Vacuum to waste — After the algae is dead, vacuum the debris directly to waste (not through the filter).
5. Clean the filter — Algae clogs filters. Clean or backwash after treatment.

Green algae is straightforward to kill. A proper SLAM will clear it in 2-5 days depending on severity. The key is not stopping early — if you stop shocking when the water looks clear but the algae isn't fully dead, it comes back within days.

Mustard Algae: The Stubborn One

Mustard algae (also called yellow algae) is much harder to kill than green algae. It looks like yellow or mustard-colored powder on the walls and floor, usually in shaded areas. It brushes off easily but comes right back. And here's the worst part: it can survive in chlorinated water. Normal FC levels don't kill it.

Mustard algae is more common in Texas than in cooler climates because it thrives in warm water and intense sunlight. It's also more common in saltwater pools because the lower average FC levels give it a foothold.

Why Mustard Algae Keeps Coming Back

• It clings to pool toys and equipment — Brushes, vacuums, floats, and even swimsuits can carry it. If you kill it in the pool but not on your equipment, it re-infects the pool.
• It hides in the filter — Mustard algae can survive inside DE grids, cartridge pleats, and sand. If you don't clean the filter during treatment, you're re-infecting the pool every time the pump runs.
• It's chlorine-resistant — Normal FC levels (2-5 ppm) don't faze it. You need to reach shock level AND maintain it for the full SLAM duration.
• It loves shade — It grows in areas with less direct sunlight, where chlorine degrades slower but the algae can still photosynthesize.

How to Kill Mustard Algae (For Good)

Mustard algae requires a more aggressive approach than green algae:

1. Test and balance — pH 7.2-7.5, TA 80-120. Same as any SLAM.
2. Brush EVERYTHING — Walls, floor, steps, behind ladders, around lights, inside the skimmer. Mustard algae hides in places you don't brush.
3. Clean ALL equipment — Soak brushes, vacuums, floats, and pool toys in a 50/50 bleach solution for 10 minutes. Wash swimsuits in hot water. This is critical — skip this and it comes back.
4. SLAM at mustard level — Mustard algae requires a higher shock level than green algae. Use the "mustard algae" column on the FC/CYA chart, which is about 1.5x the normal shock level.
5. Add yellow-out or sodium bromide — These products specifically target mustard algae. Sodium bromide converts some of your chlorine to bromine, which is more effective against mustard algae. Follow package directions.
6. Run the pump 24/7 during treatment
7. Clean the filter — After 24-48 hours of treatment, clean or backwash the filter. Soak DE grids or cartridges in a bleach solution.
8. Continue the SLAM until you pass the OCLT — See our SLAM guide for the overnight chlorine loss test.

Texas-Specific Algae Tips

• Start early — In DFW, algae season starts in March. Don't wait until your pool is green to start maintaining FC levels.
• Monitor CYA — Texas sun destroys chlorine fast. Keep CYA at 30-50 ppm. Higher than 60 and you need more chlorine to be effective — and you'll need even more to kill mustard algae.
• Brush weekly — Even when the pool looks clean, brush. Algae starts growing on surfaces before it becomes visible.
• Check shaded areas — Mustard algae starts in shade. Check under the diving board, behind ladders, and in the skimmer.
• Clean your filter regularly — A dirty filter is an algae breeding ground. Backwash or clean monthly, or as part of your weekly service.

Don't Want to Deal With It?

We clear green and mustard algae every week. Our green pool recovery service handles the full SLAM process, all chemicals included, with photo reports. Starting at $299. And if you're on our weekly service, algae should never be a problem — we maintain proper chlorine levels and brush every visit.

Call 682-399-2593 or get a free quote online. Se habla español.