The “foam” is just steam.

A microwave heats foods by very quickly vibrating water molecules. This quick vibration causes heat.

As small areas of your tea suddenly become hot, they turn to steam; but since the surface of the tea may still be cooler than the rest, the steam condenses into a surface “fog”.

It is similar to the effect you get when you place a pot of water on a burner and then heat the water to boiling. Before the water boils, you’ll notice bubbles form at the bottom, start to rise but never reach the surface. The cooler water near the top of the pot causes the steam to re-condense. When the water at the top finally is hot enough, the steam bubbles out and the water boils.

The bubbles in your tea form the foam, because your tea is not “pure” water, but has the added components of tea.

I think it’s just air from the tea bag bubbling up in the water–nothing to worry about.

I would think it would be containments in your water making the foam, not the water itself. Maybe try filtered water, see if that helps.

not sure but i believe this will help you see things more clearly

i would avoid the microwave and boil your water.