Brassia Rex disease/insects on back of leaves?

[NYCorchidman]

The pictures are not clear, but it appears that you got a serious case of spidermites.
To make sure that is the case, run a white paper tissue on the back of the leaf. If color is apparent on the tissue, then it's the spidermite.

You can run a wet cloth (fine cotton type) along the underside of the leaves to remove majority of the adults and some juveniles.

Then wash the leaves (with special attention to the underside) with soapy water.
This will kill or detach some that are still on the leaves.
Then heavily mist (don't use garden hose as that might injure the leaves unless the water pressure can be controlled properly) the plant back and front to wash off.

You will have to repeat this on three days interval for a couple of weeks to be safe. This is necassary because the eggs continue to hatch and any surviving adults will reproduce quite fast under certain conditions.

If your plant is NOT in very open mix, then do not rinse down the entire plant because that might ruin the roots by keeping them too wet.
What you can do is just diligent washing (with damp cloth) of the leaves only.
Even after that, check the underside of the leaves often.

Also, plants that are stressed due to underwatering tend to attract mite more, so water your plant well and do not leave them bone dry for too long.