What is this black squishy algae?

[RickvDam]

Hello,
Today while doing a waterchange I found this black stuff. It's soft and squishy but can't be blown off. It's really stuck on there. It's on an empty shell.
What is it and should I remove it?

 

[UMALUM]

It's just entrusting sponge, not sure of the exact kind as the black usually dictates exposure to oxygen. Most likely Caribbean and could serve as beneficial with a close eye kept on it.