Grows on glass, scrape and it is not affected.

[jerimiah74]

This grows on the glass. If i scrape with a algae scratchy, like you use on a stainless pan, it grows back. The tentacles come back. Tank is 7 months old. These creatures showed up about 2 months ago. It can be scraped off with fingernail, but the algae scratchy doesnt affect it. Started with dry rock. Have made purchases from lfs, and liveaquaria.

 

[Lovemyreef2015]

Looks like a type of Foraminifera. Similar to Marginopora vertebralis. No harm to a reef tank. Most don't live long in our systems.