They are considered to be harmless. If you do a web search for mealworm colony mites you'll run across many pages about how to deal with them.
Some feel that if you completely dry out the bedding, remove all food for a length of time the mites will die out. But this may take a very long time and never happen.
Others believe starting with a new tub, new bedding and transferring the mealworms over will work ok.
And yet others say just destroy the entire colony, don't risk contaminating a new container. From what I read, if your mealworms are kept in a warm humid climate the mite infestation will quickly grow and they will start killing your mealworms.
No idea which is true, I've yet to run into a grain mite infestation. Had grain moths once and that was bad enough.