That sounds, or looks rather, like a pretty well rounded collection of snails. Phosphate is one of the many minerals you'll get to know when keeping a reef tank. I could go into a long and confusing description of it, but all you really need to know is that phosphates feed algae. The good news is its an easy problem to fix. All you have to do is add some phosbuster in a media bag to your filter or sump. The other thing you'll need to do is test your source water. This is sort of contraversal since you get your water through a store, and I'm not saying anything bad about any store! Its not unheard of for RO systems, although one of your best bets, to have issues with the constant flux we get here locally from our tap. Certain areas are better than others about this. The other thing I'm going to recommend is if at all possible try to avoid doing a water change after a heavy rain. A lot of ppl have come to the store and mentioned noticing more Phosphates after it rains. Here is a good source if you'd like to know more...
phosphates
-Kat