I keep mine on the sandbed cause thats were the best flow for galaxeas are at. You wanna see the tentacles bounce or sway back and forth. I do notice that my galaxeas detest high flow and high lighting. Honestly, if you have kept torches/hammers/frogspawn you’ll know what a Galaxea likes. They’re similar (since they are related to Torches especially) but personally much easier to keep. They tend to shrivel up due two things besides poor water chemistry
1. Way too much flow, especially if its not chaotic/turbulent and is only hitting them from one side. You really gotta play around with your powerhead, there’s no easy way to get it right. They’ll retract very much like yours.
2. Way too much lighting or too little. Too much, they’ll shrivel and worst case bleach. Too little, they’ll brown out and you can’t see their fluorescences. By right bottom to medium placement in your rockwork should work fine.
Regarding, the mucus. This is news to me as in my experience with galaxeas even growing a colony from a single polyp (first picture) I have never seen my shed mucus.