The answers I see are good for when you have mold/mildew on the grout of your ceramic tile. If it is from a leak in the wall, that is dangerous and needs to be checked and probably removed and replaced by someone who does tile work who can also remove that safely. Breathing in some kinds of black mold is dangerous.
If it is around your register (for a/c or heating), it can be caused by the a/c running a lot and from the "boot" within not being insulated. The boot can be replaced if it is too icky so you won't have that again (said my hvac repairman last night).