One of my live rocks has been turning a dark. It's not a slimy algae. I wanted to know the consistency, so i tried brushing it off, and only came off with scraping. Is this coraline of a different color or am i getting cyno? It is is only one 1 rock out of 130#'s. It is only growing on one side( about 1/6 of the rock). Is this good or bad??