It's my tank that Jtrux is talking about. I've only had a couple of saltwater tanks and still consider myself somewhat novice, so I might have missed something obvious. It’s a FOWLR 75 gal with a DIY 20gal sump, Koralia 2, skimmer (made for 100gal) and standard lighting. It has approximately 60 lbs of live rock and 3 inches of aragonite. My ammonia, nitrite, nitrates are all 0, SG is at 1.0225 (salinity 30.5 ppt). My calcium has been low due to a water softener but I use additive to get it back to acceptable ranges. I have not checked phosphates, which probably caused this. The tank has been set up for about 5 months, cycled and had fish in it for 3 months, but this just popped up about 2 weeks ago. I have probably been over feeding slightly while trying to figure out what the CB butterfly likes. I am going to check phosphates but in the meantime is there a chemical treatment that does a good job?

What is the worst that this stuff can do other than look bad?