Star polyps and clamshell questions

I have a reef and fish tank with various soft corals....mushrooms, polyps, leathers. The star polyps seem to either die off or disappear (maybe eaten?) after having thrived for months. This has happened with several specimens. No problems with any of the other corals, including yellow polyps. sea mats, button polyps, etc.

Any ideas about what's going on, or any similar experiences?

Also, one of the star polyp colonies was attached to a large clamshell (about 3 inch diameter). Is it ok to just leave the clamshell (now that the polyps and the algae base are all gone) in the tank...or will it somehow foul the water? I don't have any hard or stony corals.

Thanks.

No nibbles apparent on shrooms. Tank has tangs, flame angel, mandarin fish, snails, cleaner shrimp, hermits.