what is this redish purple stuff on my rocks Good or Bad

[Buercky]

Im in the process of cleaning up my tank and wondering what this is that is growing on the rocks.

 

Thanks for the help

 

 

[Reef Hollister]

My guess is a type of gelidium. ReefCleaners.org has a section on nuisance algae, in which this is addressed. I've had it before, in fact, still do right now. If it bothers you, remove the rock and brush with hydrogen peroxide. It will then turn a bright orange in a day or two as it dies off.