Is that bad
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with regards to that shell, maybe take it out. Not sure if this scenario is likely, but it may slowly dissolve, and add phosphates to the water. Not a huge issue, but why deal with it?
Also, it takes up precious space that you can use for coral growth.
BUT ultimately, it's your aquarium, if you like it keep it, just (as with all my advice) maybe google if it's OK to have in there.