These tiny arachnids feed on the chlorophyll in plants. They are so small, it takes a microscope to see them. However they do spin webs. If you can see their webbing, you’ve got at least 2 generations of infestation on your plants my friend. And they breed crazy fast.
Mites usually reside on the underside of leaves. High humidity and a room temperature of 70-80F (21-27C) is what keeps these mites happy. So to combat this, keep the grow room cool at about 60F(16C) with humidity around 50%. This will hopefully slow down the infestation rate and give you a chance to enact other measures.
Neoseiulus (Amblyseius) californicus are predatory mites that you can introduce into the greenhouse that will prey on the spider mites. After they do their job and eat all the spider mites they will die off. Another option though would be to use a spray instead of the predatory mites. Neem oil is seems to be very effective. I recommend to spray three times 5-10 days apart. That should do the trick to get rid of all the spider mites if you are consistent at keeping the grow room clean.