Weird stuff in planter

What is this weird stuff in my planter?  It is powdery, and water runs right off of it.  A fungus?  Mold?  Can I leave it be, or do I need to remove it?  Since it is powdery, do I need to wear a mask when removing to avoid breathing in potential spores, or is it harmless?

Looks like a type of slime mold, although doesn't look like dog vomit mold.