As for description... They're about 0.5mm if not smaller... maybe even 0.2mm. They're white. Thanks for the responses so far!
Wow! It sounds as though you have your own little ecological experiment going on there!
If they are round and have legs, they are almost certainly mites, as Peter suggests. If you can, look at one under a good magnifying lens and compare it with pix of mites on Google (see below!) Images. Count the number of legs per bugger!
Their behavior is a bit odd for mites, though, they should be lurking in the substrate (could you tell us what that is, please?), not gallivanting around yr tank walls.
My guess is that they are thoroughly uncomfortable. If you have condensation on yr tank walls, it sounds as though you have 100% humidity and a very moist substrate. Mites tend not to like such conditions (neither do their prey), so they might just be trying to escape the moisture. Of course, they will probably starve to death at the top of yr tank (or drown in the condensation!), so that should solve the problem sooner ot later!
I guess that they could also be the larvae of some tiny insect that are getting ready to pupate (in which case they will only have six legs). You will know if you find pupae.
Please take a closer look and tell us more!