Could be a gorilla crab that hitched hiked its way in. Could be urchin, sometimes they are clearing coraline, and decide to consume coral too. Could be emeralds, they can go for sps polyps, in cases where other food source is lacking. Could be a different pest, like RB's or AEFW. If struggle is only with acros, not BN, monti, or other sps, then look to AEFW. Could be flow, but I doubt it, most pumps have wide flow patterns, and in the wild, they are bashed with flow
Finally, it could be a biological thing, and I have first hand experience with it, took me a year before any sps would live.
Finally, it could be a biological thing, and I have first hand experience with it, took me a year before any sps would live.