Maybe due to nutrient build up over the last few months. Stuff tends to accumulate and cause problems if not removed through water changes or other means. When you do your next water change, try about 10%, take out the effected rocks and clean the nasty stuff of with qtips or a new toothbrush. I just had to replace a couple rocks because nothing I tried would stop the algae from coming back. I'm pretty sure they were leaching phosphates and would not get better anytime soon. If you collect your own rocks look for some deeper water choices with lots of coralline on them. I find the nasty stuff tends to not grow on rocks already colonized by the coralline. Also shake them off tons underwater and try to scrape all the stuff that might be growing on them while still in the ocean. It will serve to reduce die off and reduce misc. crap coming out when you put them in your tank. I've gotten some really nice pieces using this method and with almost no nasty stuff and no curing issues. Some of the pieces I've gotten from OAC have issues with brown nastiness growing in places. I really don't know why but maybe they weren't colonized enough with the good stuff when I got them, allowing for nuisance algae to get a foothold.
Also I mix my own water using RO/DI and the Coral Pro salt at petbox. It's pretty much a ripoff but I haven't had great luck using water from the ocean. What kind of filtration are you using?
