Welcome to RAF! Would a hang on the tank bio-wheel filter be ok to use instead of a skimmer? I think you are talking about two different things. A shimmer is met to put things out of the water and a bio-wheel filter is something else. Are you going to use live rock for berlin system? I would suggest...
It doesn't look good by looking at the picture but time will tell. If it start to receed it mantle back into the shell, then it is gone or going. I noticed you have it very close to some corals, maybe it got stung.
When you went with the new lights, are they the same Kelvin R? The new bulbs will be stronger so the clam will have to acclimate to the new intense lighting. You might lower your photo period down and slowly raise it back to where you had it over the next weeks.