If they actually are spider mites, then they're just a plant pest and should be harmless, though you might want to check all your plants closely. By the time people notice spider mites, they've usually wrecked whatever plants they were on.

It's pretty likely they're actually something else, though. With your moss, I'm sure there are a variety of little arthropods crawling around which are no threat, but still will end up on the toads. This tiny species of springtails always make me do a double take. Can you give any more info on them or if they seem to be actually seem to be biting/bothering her? If you have your old tank still, you could always set up a quarantine tank while you try to figure out if they're a threat, if only for peace of mind.