Agreed....JerryC has given you good advice. I have some green along with the purple on our live rock. It's another form of coraline algae if it's hard. It's actually quite pretty, but will eventually cover over with the purple coraline.

What Jerry is saying by the "soft" kind, is the green slimy kind causing a coating that scrapes off. The same as the algae that builds up on the glass in a tank that your snails help keep under control. That is typical green algae that you don't want.