What is it?

[fredy free]

[/QUOTE]Two questions; I had this (attached picture) plants/algae show up a month ago it is hard as if calcium based, green in color as pictured, the rock it is on was very aged when purchased from a shop going out of business over 5 years ago and the last thing added to this tank was snails and that was over a year ago. What is it and is it desirable or foe?




Second question, in photo you can see the rock is made up of a mass of less then 1"oval or round what I assume was living creatures? Years ago when I taped this one small rock it was as if you would tap a snail attached to a rock, thing is it has not moved from the same spot after years still if you tap it is attached to the rock like a snail? What is it? Rock may have grown not sure? Thanks for looking.

 

[saltwater sam]

That's Halimeda. Just don't let it get overgrown and you'll be fine. It can go sexual if you let it grow too dense, though.

 

[Fish_BowLisha]

Perfectly reef safe. Some tangs can eat it even though it's calcareous. Apparently it's good for their digestive tract. ( maybe saltwater corner quote? Not sure so much reading this past few days lol)