There are some polar molecules but not the ones in which the molecules would rotate efficiently with microwave energy.
I was thinking of the wife and the kitchen.
Let's say it did heat up. With the cap on it might explode and cause a fire.
With the cap off it might gurgle or percolate up and over the top and spill. "Honey, what's that smell coming from the microwave?
Solar heating or setting over a hot air register, IMHO, is the better way of warming it up.
I think what you are seeing at the bottom is a very minute amount of potassium borate, sulfur, and some thickener settling out of solution.