My hermit crabs skin fell off and is all pink?

Congrats! Your hermie has just molted! Hermit crabs have exo skeletons so in order to grow they have to shed their skin.

tHIS is a VERY stressful process for them, so never interupt them. They dig underground to molt, so they have to eat and drink a lot before so, because they can be underground for weeks to months.