How to permanently remedy a patch of mold on a ceiling from a water leak above?
There will be the potential for the mould to reappear as (as you have correctly surmised) there might be still some water in the ceiling space.

If you can, lift the floorboards in the upstairs room. This will allow you to see the extent of any damage and also get the air into help dry the patch out. If the area affected is not too wet then leaving the boards up for a while will help. If it is really wet then you might have to cut away the damaged board and repair the ceiling.

There are other questions here on how to patch a hole in the ceiling.