I don't know of any remedies for stings from other corals. The best suggestion I can offer is just try to keep it nourished with zooplankton and/or small bits of krill, clam, squid, etc., and if you're not running carbon, I'd strongly recommend it. If zoas or palys are releasing toxins into the water, that may be causing it to further decline. The carbon will remove those chemicals. Unfortunately, there's never a guarantee once a coral has been adversely affected by a different species, but if you can keep it eating, hopefully that will help.