HCA: Hermit Crab Association

Hi!
We are a newer crab family. My daughter purchased one (Mr. Krabs) in July at the boardwalk and after research we realized there was a lot to know about them and they couldn't live in the mesh cage we got with purchase. Since then we purchased another one (Pearl) from a pet store. They are now in a 10 gallon tank with coconut fiber substrate about 6 inches deep. Mr Krabs has gone through a molt and Pearl seems to be doing well. My daughter is on a mission to save all the hermit crabs from terrible conditions. We were in VA Beach over Columbus day and she found that a souvenir shop had 10 crabs left a the end of the season, she picked out a smallish one that was incredibly active. We brought LilB home and introduced him to our current crabs. He was very active for a few hours and then he went under the substrate and we haven't seen him since. It has been about 2 weeks. He did not eat anything before going under and the conditions he came from were not the best. Should I be concerned yet? Or assume he is still destressing or molting. HELP!

Welcome to crabs and to the site!
Assume he is molting or de-stressing. Don't dig to look for a crab underground unless there is an emergency like a bacterial bloom or a flood. They can stay underground for a few weeks or a few months even, depending on their size - and sometimes they might come up in the night or when you aren't watching the tank. I am of the opinion that if a crab has passed away underground, there is nothing to be gained by digging it up - but if a crab is in a critical stage of molting, there could be serious and even fatal consequences of digging it up. So better to wait it out!