re: Mold vs Mildew in a Rental

The only way to know for sure is to have it tested. Even if it is mold there are several types. Nothing will kill it permanently except removing the source of moisture. You can spray whatever on it but if the moisture source is there it will come back eventually. There is probably a leak in the wall or water running out from a bad seal in the sink. Dealing with the moisture source should be the main issue