Pinhead cricket?

Yeah pretty sure that is a cricket. Definitely not a mite, isopod, or springtail. Maybe it hitched a ride on a bigger cricket?
Cricket-ception! 0__0
That's pretty wild that they start so small.

Yes, it is. A female that you put in probably laid eggs in the soil before she was eaten. I cut the ovipositor off before putting them in to avoid eggs being laid.

Yeah since I picked that one out of the water dish I spotted 2 more hopping around. I'm not sure what to do? "Humphrey" my T. apophysis is sharing his house with some of my isopod friends and now I got unwanted pinhead crickets.

Should I just clean out Humphrey's enclosure (and salvage his isopod friends) and replace it with clean
Substrate?

@Stella Maris No, they will die off quickly enough without food, or they will escape and die almost immediately. I had it happen to me some time ago and I did nothing. I wasn't overrun with pinhead crickets (I kinda wish I was, they would have saved me a bit of money).
It is not a big deal.

Just keep a keen eye out. Probably fine if you just let them die off. I'm sure the mortality rate is pretty high, but if any of them get sizable, definitely remove them. Better safe than sorry. (1mm or up, they need lots of protein in their diet, e.i. your T or your isopods, to grow properly and will not hesitate to take a bite) A trick I used to get rid of unwanted feeders back when I had Green Anoles was to place a food sorce for the feeders in the enclosere, and every time I noticed one eating on it, I would pick it out before it could go back into hiding. I used orange or apple slices usually. I don't think apple slices would bother your T, but orange might? Not sure, just watch out for mold if you try this.
Hope this helps!