Hmm.. stray voltage from a DC pump bad enough? I thought that would affect all corals, not just acros.
Anyway, can you check if there's any water parameter (PH, temperature, etc) swing? Although some amount of Ph swing is expected, check both Ph and temperature swing - morning, day, evening, night. Do you have a PH meter?
Other more uncommon factors are - toxins (heavy metal, copper, or from other corals) or disease/parasite (AEFW, etc).. at least check thoroughly on acros dying to see if you can spot anything obvious..