My hermit crab is digging, is he going to molt?

Maybe and maybe not. Hermit crabs dig for a lot of reasons and sometime just because they feel like it. Sometimes crabs, especially newly-purchased crabs, will bury down into the substrate to de-stress before coming back up and saying hello. Please do not disturb a newly-purchased digger. If a hermit crab is too cold it will bury into the substrate in an effort to keep itself warm. The ground is warm in the wild tropics where hermit crabs live. Please don't let your crabitat stay below 72 degrees Fahrenheit for more than a day or two at the most or they will slow down, get lethargic and eventually die if the temperature is not returned to a more comfortable temperature. Some crabs just plain like to dig a lot. Ecuadorians and C. cavipes are known to be very big on digging. There are people who only see their cavipes crabs at night because they are dug in during the day.  After you get to know your crab's personality, you will recognize if the crab is a digger by nature and will know when it is digging because of molting or illness.