Are my hermit crabs about to molt?

i would have to question your substrate ... the absolute worst to use is calci-sand ... it will start to stink and mold and is one of the worst substrates and not suggested for hermit crabs ... tropical soil sounds questionable too ... the substrates suggested are eco earth (coconut fibre) and/or play sand ... when you first get hermit crabs home they can need to destress (or change tanks) ... in which they will bury themselves but come up daily for food, water ... if they bury and remain buried then they are probably molting ... but do know that a hermit crab can take several weeks to molt and longer for crabs bigger than yours ... so i am sure you would notice a crab missing for a month :O) the crabs i have (purple pinchers) have all molted while buried and ate their exoskeleton while buried ... and i have had several crabs molt at the same time (i hate that :O) ... and you should have two pools of water, fresh and marine ... all hermit crabs require access to both types of water and made with fish supplies not hermit crab supplies ... when they molt sometimes for a few days before they bury they will eat a lot, drink a lot and become less active ... and if your guy is molting be sure there is a form of calcium for him to ingest when he comes up ... helps harden their exoskeleton ...