Some of my pots have gotten that too, mostly the ones with peat-heavy mixes. I've always thought that it might be some type of slime-mold(actually a type of protist, I think) or related group. I've tried fungicide on the stuff, which hasn't seemed to have helped very much.
The growth hasn't directly attacked any of my plants, it just stayed on the top of the soil. If you don't like the look of it, you could repot into a better draining media, or stir up the top layer of soil a bit. Both of these methods have worked for me.
