We'd need a picture, marine life is far too diverse.
It could be hair algae, if there's no or few other hermits to clean it's shell.
Although not a hermit, decorator crabs stick all kinds of rubbish to themselves.
Pom Pom crabs have tentacles on the ends of their arms, they're actually tiny Anemones it carries around as boxing gloves.
See if you can get a picture, I'm curious to see what it is now.