You won't find spider mites OR beneficial predatory mites in your worm bin. The beneficials predate on spider mites, which will only be found on green plants.

You have some sort of worm bin mite, about which know pretty much nothing. BUT with one click into Google using 'worm bin mites', I found you a bunch of articles to read. Click on google images first to see the duplicate of your picture. Which was excellent, by the way.

By the way, there are many, many species of mites and they are found in all kinds of habitats. Each tend to be very habitat specific, meaning that you can often come pretty close to identifying them by observing where they reside.

Here is a link that might be useful: Worm Bin Mites