Little Spots of Green - Algae, Seaweed(!), or What?


New member
Apr 2, 2020
Hi folks. I live at Amelia Island, just outside Jacksonville FL, and my pool is just meters away from the salt marsh. Pool is in-ground 15,000 gallons with salt water/chlorine generator. I test it every week and keep chlorine, Ph and alkalinity levels where they're supposed to be. I don't have numbers to give, but use the dye-based system to stay in the right color range. Anyway, this summer I noted in just a few places dark spots, about the width of a pencil eraser. They wouldn't vacuum up, so I tried to brush, and they were very resistant to that. Finally I just got in the water and scraped a couple off with my fingernails, and it was a dark green, fibrous material that looked a lot like seaweed. I've read about mustard algae but the pictures I've seen on-line don't seem to represent this. The pool is otherwise nice and clear and as I said, the basic chemicals always test fine. Any thoughts about what this is, and how to treat? Thanks!


Gold Supporter
LifeTime Supporter
TFP Guide
Jul 3, 2013
Southern OK
Pool Size
Salt Water Generator
SWG Type
CircuPool SJ-40
Welcome to TFP :)
The green is probably some kind of algae as there are many kinds and some are tougher than others.. You can take your pool to SLAM level but you need a DPD test kit to do a SLAM Process

It sounds like you have been very lucky as SWG are easy and it keeps your pool out of trouble.. You really need to get a great Test Kits Compared as it will allow you to know exactly what is going on in your pool..

You should know exactly what CYA, FC and CC you have at all times.. Take my pool, I have 7 FC, CYA at 80 and 0 CC and salt at 3800, SWG happy and creating chlorine...


Silver Supporter
Aug 22, 2017
West Palm Beach/Florida
Pool Size
Salt Water Generator
SWG Type
Hayward Aqua Rite (T-15)
Sounds like classic black algae. You should get in the pool and get them all scraped up. I use a bamboo BBQ skewer to pick at them. A wire brush works too. Then get a 1 gallon pump sprayer and fill it with LC. Spray the scraped up spots with that to really help kill the roots.

Now you will need the test kits that Casey mentioned because you will have to run elevated FC for a few weeks to really make sure those spots are dead.
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