I can't say for certain it's a spider mite but appears to be something. Could just be debris.
Far as the ziplock check once every 3 days for any signs of budding and for any type of fungi/mold.
Regarding pesticide if you go, go for the throat! Lol!
Amidst the many caveats of a grow room one of the benefits is you can go as hard as you want on pesticide in my opinion.
One of the main reasons broad spectrum pesticide use is "discouraged" are the detrimental affects to the good insects and possibly the wild life.....none of which exist in your basement.
Any residuals can serve a dual purpose as a deterrent to any bug wanting to take up residence! Lol!
I used to have a hygrometer I think it is called but now I just go my the water droplets on the side! Lol!
Far as temps once your active cuttings are deposited into the grow chamber, high 70's is optimal to my knowledge but ambient air and soil temps are not the same so if you aren't heating the soil then will seek to have a hotter air temperature which is the route I am taking bc I don't have any mats! Lol.
But it's all relative. Keeping in mind the humidity keeps the excess heat in check by ensuring the cuttings do not dehydrate. Vice versa the heat keeps the excess humidity in check by quickly evaporating it. The high sand content keeps the high amount of surface run off in check by not holding much if any moisture.
Crap you learn through trial and error! Lol! So a final word of caution with high rh. Make sure you have something in place to evaporate it and or transfer it out!
It's not as important with active but un-bud-broken cuttings but once you get foliage seek to evaporate it off the leaves and roots if you plan on growing what you root in the same pot.
Personally I prefer to transfer them out as soon as I see the first signs of rooting. As seen in the pics the crazy high rh is for the overall rooting process. The places they grow have much less. Approx high 60 to low 70 rh.
I had a couple heat mats but they quietly died. I'm guessing they must have a thermal fuse in them. If I get the time I will perform a post mortem bc I can easily repair them if it is just a fuse.
I will get some for next winter...no point now! lol. Also with the relativity factor adding a heat mat changes it all. Meaning I'd have to amend my media to hold more water.
The old axiom is definitely true here "if it ain't broke; don't fix it". It's a little too late for me to experiment. I prefer to do that in the beginning of the cool down so I only have a small control group of test subjects to try stuff out on and my high value vegetation can still remain outside but covered if need be until I am satisfied!