They definitely will be feeding on the zooplankton. From my experience with them, they tend to feed dusk and night, so switching your feeding schedule to primarily day time or late morning will also diminish their proliferation. I never had any problem with them actually irritating or damaging my corals. I found they tend to stay on the underside of rocks or plate corals and then progressively come out as it gets dark. I had one from 2013- late 2015 that grew quite large, sweepers over 20" by the end of its time in my tank. Never irritated the plate coral it was on, even when they grew to near the same size. I simply fed the plate coral during the day. Sold both of them in late 2015 as the jelly always attached to the plate underside by morning. I've had far more irritation to my tank inhabitants from vermitid and other filter feeders than I ever had from any of these. They are
large, but I found it more as an exciting oddity than a pest problem. I never actively fed them and the populations grew and diminished besides the large main one which was stably hosted by the plate coral. Perhaps they were being consumed by one of my fish/shrimp though.
Attached are some photos of the large comb jelly (my first one) from February 2014 on the plate coral, and the plate coral during the day with the jelly hidden underneath.
Are you finding that the zoas are regressing or lacking proper extension during the day?