I've been battling this stuff for about 2 weeks now. It looks and sounds just like mine, which is definitely cyanobacteria (slime algae). It is dark purple and when the lights are on, it forms small bubbles under it, which is oxygen (it's byproduct). It has been very tough to eliminate. I'm still trying. It has been reduced since I've added 2 more powerheads and have cut way back on my feeding. Unfortunately, I think it killed one of our corals. It formed on the base, and I got it off once I noticed, but the coral hasn't come back out since. It also seems to have a shininess to it that it never had. (It's a leather coral that has done great up till now).
Good luck on yours. It's nasty stuff.