what is the purple stuff in rocks
The purple stuff is coraline algae, which is a kind of algae that is desirable to have on your live rock. It grows where you have good light and good water conditions. So keep taking good care of your tank, and the purple stuff should spread. Sometimes it's more pink than purple, which is also good. Sometimes coraline algae also grows on your glass, where it looks messy unless you scrape it off.