If it's a small patch I would haul out the rock right now. They're a pain to get rid of. And trying to scrub it off in the tank will just spread it. I've tried turbo snails and they wouldn't touch it but I've heard others having success.
If it is like a red-slime coating.. you may be able to just siphon it out. I have used airhose stuck into the rigid airline. Hooked it up to an aqualifter. Plugged the thing in and had a mini-vacuum. Works like a charm for smaller jobs.