I've had this stuff in few patches for a great while. It's starting to get bigger since I've controled my prams better with a calc reactor and Kalk drip. This is the size of a quarter. Nip it in the bud or let it be?
Rich
It looks like foraminiferans to me, but there's lots of different kinds (I have at least two very different kinds in my tank)... take a look at this page: http://www.reefs.org/hhfaq/pages/main_pages/faq_rock3.htm , near the bottom, and see if the pics and description match what you have.