What IS this?

[christyf5]

no, bryozoans are something completely different. These are foraminiferans:

Foraminiferans These little creatures are basically shelled protozoans, one celled organisms (Subphylum: Sarcodina - the amoebas). They form shells primarily composed of calcareous material, with siliceous material from sponges, or other organic material. They are mostly sessile (non-moving), but can move with the use of temporary structure called a "pseudopodia". This little "arm" reaches out and pulls the rest of the critter along. In any event, they are harmless bacterial/detrital consumers. There are many different shell shapes and colors that these guys may have, see more foraminiferans

from: http://www.reefs.org/hhfaq/pages/mai.../faq_rock3.htm

I have both kinds in my tank. They're pretty cool