Help with a green thing please

[hugothedog]

Have noticed a few of these appearing on my live rock and dont know what it is, can anyone help.

It appears to grow slowly (over months) and then a hole appears as if something has hatched.

any ideas?

http://photos.yahoo.com/[email protected]

 

[Anonymous]

That looks to be a sponge, it is harmless and a sign of a healthy tank!