I asked my serious saltwater friends what they thought this critter was, and I was told it's very likely a Foramoniferian. It appeared on the walls of my frag system, and seems to be filterfeeding. It doesn't sting anything, and didn't turn into a plague. Just an interesting reef curiousity. As I cleaned the front pane of my tank after these pictures, they came right off. I left some on the lower back wall since they aren't disturbing anything.