That is velvet. The problem with velvet is that it looks like ich for the 1st week and a bit the fish get it, and by the time you figure out that it's velvet it's usually to late to save the fish with treatment. When I had it I started with 34 fish and ended up only being able to save 8 fish
Here's a link to thread I started when I was dealing with velvet.
http://www.canreef.com/vbulletin/showthread.php?t=51938