another newbie question

[AJ_77]

Hey, that's the same stuff I've been wondering about lately as well, and it fits your description to a T. At first I thought, "burgundy coraline?" But it does have a slightly velvety texture, but I can't scrape it off with a fingernail.

I'm hoping its cyclical - some of it seems to be receding, and its only on one or two rocks at one end of the tank...

It is almost like a thin plating sort of cyano??