Anyone know what this is?

[sgd17]

And how to treat if bad?

 

[lapin]

very cool
It could be a plating sponge. I have never seen a green one.
https://www.qm.qld.gov.au/microsites/biodiscovery/03sponges-and-corals/diversity-of-sponges.html
or
https://www.marineplantbook.com/marinebooklobophora.htm

 

[JimSRT4]

It looks like calcareous algae. I had something like that years ago when I ordered Florida LR.

 

[Biocube32]

Huh, interesting. Looks kind of cool