I'm about two years late, but I recently found these I my bonsai pot as well. I'm in a soil science class right now and they are called orbatid mites. They feed on small fungus, algae, dead plant tissue and nematodes and springtails. They're major decomposes and can be found everywhere in nature. If you see these it usually means there's too much water that promotes fungus and algae. My own bonsai was overwatered when I found them. They won't hurt your bonsai.