I had the same problem on my 210 gallon reef. I started the tank in July of 2009, and used a silica based sand. Everything was good (after the cycle) for a couple of months. Around February of this year the slime started. It was awful, it covered and killed a large portion of my corals, it covered the sand, it covered the rocks, and when my cleanup crew ate it they died! I had enough and decided to remove the silica sand with calcium based sand (having read that excess silica can cause this particular nuisance). I noticed a difference immediately, and was very happy for about 3 weeks. Unfortunately, it came back with a fury! It was twice as bad, and I just couldn't get rid of it. I was on the verge of starting over or getting out. I decided to try a couple of things before I did something radical. Number one was to change my salt. I was using Reef Crystals, and changed back to Oceanic (which I used in my old 55 with great success). This really helped a lot, but the slime remained on my rocks, walls and corals. I then removed the canister filter I was using (for charcoal/Gfo only), and I changed my bulbs from 10000 K to 20000 K. I also performed water changes every 3 days, removing as much of the slime as I could. Finally it appears the problem is solved. Its been 2 months and there is no trace of the stuff left. I think the new bulbs were the tipping point, and maybe installing new bulbs might help your problem as well!
