Please help

[Bill]

This algae looks familiar and shows up in my tank on occasion. In fact I have some in my tank right now but it's starting to disappear.

This might be a long shot but are you running a calcium reactor? If so, you may need to dial it back or better yet, just use kalk for a while. When running the reactor, effluent should go directly to the skimmer to remove excess CO2. This made noticable difference in my tank.

Get a silicate test kit and measure the fresh made saltwater. I don't think the TDS can detect silicates. Until you know your water is free of silicate, stop the water changes for a while to see what happens.

Use phosphate free carbon and replace it often. Also use GFO. Although your PO4 test shows zero, the rocks may be leeching some. I used lots of carbon - like 2 cups for 200gal. to remove dissolved organics.

Try to cut back on feeding - this one really helped me. Also, remove any filter socks or sponges from the system.

And, if possible, remove the rock and scrub algae off - in a separate container of saltwater. This way, when it dies, it wont add the nutrients back in the water.