Hi Warren,
There are a number of possibilities:
1. It has been there all along and was flipped over by one of the rover's wheels, exposing its underside.
2. Ejecta from meteorite.
3. A crystalline growth of some sort.
4. A fugal growth as you suggested.
5. A discarded scrunched up foil some Martian left behind after eating their meal while checking out the rover....
1 and 2 more likely though.
Bo