Little bit of cyano on the sandbed. In my experience GHA has a harder time growing on rock encrusted well with coralline. So if your dry rock is growing GHA and your live is not, that could very well be the reason.
You need to get your nutrients under control. You will likely get a false low reading if you test for them because the algae is consuming them quickly. But they are there. Start by pulling out some rocks and taking a wire brush to them to remove the GHA. The more you can remove manually the better, that exports nutrients. Use a turkey baster or powerhead to blow off the rocks and get all the detritus out of there. Next break out the siphon and suck up all the gunk you can find, including the cyano on the sandbed. Give the sand a little stir too if you want, it may be full of detritus also. Keep up this process for several weeks.
On the equipment side several things will help you battle this problem alongside the normal practices of good maintenance. Skimmers, refugiums with chaeto, GFO reactors, turf scrubbers. You can use none, one, or all of these to help you in this process.