**UPDATE**
Well, I ended up getting a lawnmower blenny about a week and a half ago. He's the cutest little bugger, but shows no interest in the GHA.
I actually had some time this weekend for tank maintenance, so I did a 22 gallon water change on one of my 120G tanks on Saturday, and on Sunday I did a 25G water change on the 120 that has the algae in it.
While I was doing the water change, I pulled quite a few rocks out of the tank and scrubbed them with a brush. I also re-aquascaped a bit. The only rocks that I didn't pull out and scrub were the ones that were way at the bottom of my live rock pile. I tried to manually grab some of that algae. I had a HUGE wad of it behind my live rock too. It was nasty. Probably about the size of a baseball when I grabbed it and pulled it out. Stinky too.
We'll see how things look now. I moved a lot of stuff around and spent a long time scraping corraline from some neglected parts of the tank. The side of my tank that gets natural daylight from the window has some brutally thick corraline on it. It took a while to scrape off, but the tank looks better now I think.