Half of the underside of my crawl space is clean, the other half looks dusty... is this mold?
The white on the joists?
See if it wipes off, first dry and then with some water.
It may be nothing more than dirt that has been sitting on the joists since the place was built.
The dirtier ones may have been the bottom of the stack resting on the ground before being installed.
As a cerified mold tester and with the quality of picture being taken into consideration.
Mold requires three things to grow moisture, humidity and temperature. I would think if these where a problem that the mold would tend to be black and not just limited to floor joist and not all the wood around i.e. subfloor.
However if you notice a damp or musty smell you may want to have it investigated further.
Here is a great link as a resource for further information.
Here is a link that might be useful: epa mold guidelines