Don't be alarmed if the PE does not increase. The only thing that acropora HAVE to use their polyps for is gas exchange - this is why they are out more at night, in bags or in photo boxes. Low flow can get polyps out more because the corals need more surface area to exchange gases with slow moving water. This is different than LPS or other corals, so don't confuse studies or papers that talk about all of the food catching that "corals" can do.
Acropora can use their polyps to catch food, but if it is larger stuff like cyclops or baby brine, they likely lack the ability get nutrition from it. The most nutrition comes from things caught in the slime coat like bacteria, protozoa, etc. - this can happen in the skin and polyps slime coat.
Too much PE can mean that acropora is having trouble exchanging gas - this can be from low flow, too much ammonia, po4 or no3 in the water column or some other elemtent or compound that is slowing down some biological process.
Of course fish nip... many of them. It is like the corals have telegraph wires where they send more code to each other that the angelfish, blenny, goby or tang is out and about.
I would just look for quick changes in normal PE - either way - and then check for issues.
