Good ol'e bubble algae (Valonia sp
) more than likely. I have it too. You can remove it fairly easily if you simple grab it carefully with your fingers and pull it off the rock. Be careful not to break the bubble, however. It will spread a lot quicker if the algae spores inside the bubble get loose.
I can't figure out how I got mine. Perhaps it came in on my LR
. I use RO
water and feed fairly sparingly, yet I have quite a bit of it. I recently put in some emerald crabs (these supposedly eat it) to see if they can put a dent in it.