I use a heat lamp on my tank and it does dry out the sub a bit but it is ok to use. Without it, my UTH is not strong enough to warm the tank and my basement apartment stays quite cool.
To keep humidity in check, keep a small clean spray bottle full of purified water near the tank so you can mist it on occasion. Not sure if I overlooked the type of lid you have, but if it is mesh and you decide to cover it, make sure to allow enough room for air flow into the tank. If you happen to have any aquarium supplies on hand or don't mind making another small purchase, having an airstone attached to an air pump in the water dishes helps to keep humidity up in the tank as well, without as much manual misting.
It could be possible he went down to molt, or he could just be destressing. My crabs enjoy making tunnels and hanging out in them most of the day, and come up at night to eat (or hoard food). You might try listening for you crab at night. Mine make a lot of noise at night with various clicks and clacks against shells or the glass, and one of my males enjoys "singing", which basically sounds like tiny bird tweets and is different from the deep grunting of a warning call that most call chirping. If it's qúiet at your house during the day, you might be able to listen for activity then. If you don't hear anything, don't panic. That could just mean a molt.
And if your crab has escaped, also don't panic. I've had a friend who lost his crab for 6 months and found him, still alive and well, in a closet.
I also agree you should get some marine salt to make a saltwater dish for him, in addition to his purified water. I've seen crabbers whose vanished crabs turn up because they can smell the saltwater. This will help him with molting as well.
He will likely need some new shells to change into as well, which can be found fairly cheaply on Etsy. Look for Turbo and Tapestry shells which will work for most types of crab and can be found at reasonable prices and in bulk. It may be a bit difficult to know what size to get though if your crab is MIA for the moment.
As for diet, you can feed him a large variety of foods. They are scavengers in the wild and enjoy a vast array of vegetation (fruits and vegetables) and require sources of animal protein and fats. They have good instincts about what's ok to eat and what's not. Eggshell is an easy way to provide calcium. Make sure to provide a variety of foods with each feeding, in case he doesn't like something he won't have to go hungry, and switch it up often. It doesn't take much effort to toss in a bite off your banana, or rip off a thumb-sized piece of cabbage, or a loose spinach leaf, etc. I also occasionally feed this mix:
https://www.amazon.com/Natural-Eco-Anim ... +crab+food Hermit crabs have tiny bellies and I promise you won't run out of this for the rest of the year if you portion properly.
Try not to stress out too much, you're doing a fine job already. If he is molting, it could take a week or more to resurface, depending on the size of the crab. You won't really know what he's up to til you see him again. Sorry if that's not much comfort. Hang in there.
