I got rid of Atapsia with a syringe and concentrated lemon juice (I just used the little yellow plastic lemon stuff from a grocery store). You have to inject it into the base (not the "mouth") and it's kind of tricky, you have to let it grow enough where you can stab it in the base before it contracts (which is does very quickly) then slowly, inject the lemon juice into it until it 'poofs' out of the mouth. It will cause localized acidity, so if it's right in the middle of your zoas it might be bad, but anything like Joe's Juice will do the same. Mine disappeared completely in 2 days.
My tanks: 120 Reef w/L2 Algae Scrubber, 60 Reef Pond w/floating Algae Scrubber, 40 Breeder Reef w/L2 UAS Tester Algae Scrubber
I maintain: 144 SW Reef w/L2 Algae Scrubber | 200 SW Reef w/L4 Algae Scrubber
Special knowledge: Algae Scrubbers, Acrylic Fabrication