They look like some sort of flatworm, for sure. I believe that they thrive in dirty, slow moving water conditions. They will make food for some of your fish, but can come to plague proportions, even harming coral if there are enough to blanket their surfaces. They do not like lots of water flow, and will lose numbers rather quickly under high flow conditions, along with routine maintenance. You don't want dead spots in the flow that allow detritus buildup! There is a flatworm that preys on acropora, although I doubt that this is it. There are several kinds that are more nuissance than destructive, I think.
