Frame is metal and the Jeep is stored in my garage (South Carolina), which the door is open all the time when cold fronts blow through. It looks like mold, but seriously this cannot be. If it's not mold, would simple car wash clean it off?
X2, early signs of a bit of corrosion.Looks like 'salt splash' . . . as you say, try washing it off.
If it is salt, you can hit it with some phosphoric wheel cleaner and a plastic brush and see it that removes it. It might take a little time for the acid to dissolve it. Brush on the phosphoric, let it sit for five to ten minutes and then wash it off with a power water spray. If it is mold, you can attack it with some mold remover and a brush, letting it sit again for a little while to dissolve the mold. I think it is salt, because mold doesn't survive that easily in the open air. The color and crystals seem to be salt.
It isn't mold
Have we tried wiping it off yet?
Put a black light on it and see if it comes alive-
It appears to have crystalline legs- salt/road chemicals likely-
Mold like things to be a good bit more porous than metal-