syphon the melting ricordea and all its brown slimy remnants out of the tank asap. it is toxic and can possibly spread to other polyps.
there are many different ideas on what causes polyps to melt away like that. i could tell you ten different reasons why yours may be melting and i could be wrong ten times.
one good tip i can tell you is to make sure the substrate that the ric is attached to is clean with no heavy build up of detritus or algae around the base of the polyp.
you may want to increase their flow, but not to much. i use a turkey baster to clean polyps after feeding them. in the wild, naturally shifting curents would carry excrement away from a polyp. Often, in aquaria the excrement simply settles onto or around the polyp. a turkey baster is a very useful tool for cleaning and feeding coralimorphs.
