Can mushrooms be re-heated safely?
In France, white button mushrooms are called champignons.  Before refrigeration was common, books suggested not to keep cooked mushrooms or reheat them, because undercooked mushrooms would quickly spoil.  If you cooked them and later keep them in the refrigerator, it is safe to reheat mushrooms.  Treat mushrooms as you would treat meat.

Mushrooms are mainly water, so they reheat well in the microwave.  Do it on low power. Depending on the cooking technique, the texture may change by using high power.