You put drywall joint compound that is not meant for high humidity environments.
Solution, remove all that stuff because it just holds moisture… let the area dry (i.e. - do not shower in that washroom for a day). Then use one of two things:
- Latex infused silicone caulking like DAP Alex Plus (this is paintable and water RESISTANT)
- 100% Silicone from any manufacturer (this is usually not paintable but there are some out there now that seem to allow it. These tend to be waterproof)
I only recommended DAP because of my experience with them but use whatever brand you like and are comfortable with. TEST it on a piece of wood or drywall so you know the consistency and feel. You *WILL* need to trowel/smooth it over with a finger to push the caulking into the surfaces to bond and make a seal. The trick is to WET YOUR FINGER prior to doing the swipe.
Some people put painters tape along the surfaces (as in the above picture) so do that if you aren’t confident of your caulking skills.