Too many nutrients bound up in your rocks, if I had to take a stab at it. Check your Mag levels and have your Ca and alk double checked at a LFS.

Take that rock out and put it in FW. Leave it there until the HA is gone. Treat with algae fix and let it sit for a week. Bake your rocks in the oven at 350 for a day. 10% bleach solution and let the rocks sit in that with FW for a week. Let it dry for a week in the sun. Bake it again. Let sit in FW for 2 weeks in the hot sun. Take it inside, and put it in some SW buckets with powerheads. Change the water weekly for a month. After that's done, put some crabs and shrimp in there. See how they fare. <-- I have never done this, or had to, but it's what I would do.

Or, buy some new rock and try again. Replace your sandbed as well. Remember, not all phosphates show up on tests. I'm sure there are plenty of other things that algaes can thrive on in different combinations. The ocean is the least explored, most complicated thing we have ever encountered. You always have to remember that.