Water damage in bathroom ceiling floor: how to treat and then prevent?
Bathroom floors are not supposed to leak. You may want to pay to have the floor redone properly. I assume this is a new house that you are just discovering this.

The photo looks like plastered drywall. Normally wet drywall has to be replaced, otherwise it will grow mold which is bad. Theoretically it can be dryed out, but that would not be an easy operation and practically requires removing the drywall to get full access to it, so if you remove it you may as well replace it.