If it looks slimey and easily comes off of the rock it is cyano. If it is hard and encrusted on there, it is brown algage. I'm willing to bet its cyano which comes from excess nitrates and/or phosphates in the water.
How long as the tank been set up?
cyano can sometimes pop up during the cycling of the tank.
What do you feed?
some foods can contribute too much nutrients to the tank and should be avoided.
How often do you feed?
too much food = too much nutrients
What are you water parameters (ie nitrate, phosphate)?
these both need to be 0, or undetectable.
What is your water flow?
you should be circulating your tank atleast 10x over, 20x is even fine, otherwise cyano will be able to take hold on rock. Also, good water flow doesn't leave dead spots in the tank where there is no flow and detritus (ie fish crap or food) can build up.
What is your water source? if its not RO/DI, than nitrates or phosphates may be coming from that.
Do you clean your live rock?
taking a turkey baster to the live rock to blast detritus out of it while you syphon the detritus out of the tank monthly is a good idea.
ask yourself these questions, post answers on here, and we should be able to get to the bottom of this.
some foo