Not so sure a lawnmower blenny will do the trick,,,mine never seems to touch if/when i get a "crop" of it,,,would rather "lipstick" the tank glass instead,,
pick off what you can, lightly drag a soft'ish toothbrush across the leather or whatever has it on it to remove what you can.
in my tank, lower levels and more water flow don't seem to help here,,,
maybe someone else can offer ideas/tankmates that will help take care of your HA,,my tangs,foxface,butterfly,or critters i have in the tank don't seem to like to eat it either.
water changes,getting rid of the nutrients that help "feed" the HA would help ,,,,