I don't know if it is practical for Ultra and others, but taking sen5241b's advice, I used the Stop Aptasia. I took a rock at a time out of the tank into a bucket with tank water. I put an Ott light overhead and dabbed on the treatment for 20 - 30 minutes. Then I would remove all I could, swish the rock in the bucket and return to the tank (or different tank). By the next morning, the cleanup crew had stripped all the dead RHA off! I have done this with rocks containing Acan, musrhoom and candy cane corals. I got a little on the mushroom, but it recovered within a couple of days.
I first tried it on base rock that had a little. I noticed that after 10 minutes, the slurry would start to float away (landed on my mushroom!), so I was happy to do it in a bucket. I used as small a bucket as would contain the piece.
I wouldn't give up! I have it far worse than you! Just do potions of that rock at a time, you might beat it!