I've seen these little miniture bowling balls in 1 of my frog terrariums, not sure why there are in 1 of not the other - 'cept the moisture level is higher with the one that has mites, since both houses have wood.
There is a spray you can buy at the pet shops to rid of these little critters. It's animal friendly since this is what they use to clean the (reptiles and such), dunno if indeed it's orchid friendly - But I don't see why it wouldn't be