HELP!!! What The Hell Is This Stuff???

[MMAX]

My tank had almost become taken over by this red moss like algae. Noticed it a couple of months ago, took a couple of rocks out and scrubbed them. It always came back in no time and was starting to grow over some of my corals. Today I dismantled the whole set-up. Every rock was taken out and scrubbed, the back glass which was also grown over with the stuff was scraped, a bunch of the sand which had collected a pile of it was chucked and finally a 90% water change was done. What in the hell is this crap and where did it come from? An aquarium store suggested possible overfeeding and high phosphates. The smell of this stuff is almost enough to make you puke and I sure as hell don't want to rip apart my whole set up again. Any advice would be appreciated.