What about doing what I had to do when I first got my tank and the rocks were covered by LHA.. Put them into a tub of saltwater with a power head and heater and place into a dark place. I used a closet. Leave it in there until the stuff dies off the rock and then place back into your display tank. Remove the rest from your sandbed etc by hand.
This stuff is growing on every inch of my rock. Grows on the glass, equipment & even in my overflow. I remove as much by hand as I can & let the snails do the rest. Best I can do... I'm stuck with the stuff.