How to seal a leak in a plastic tubing connection

To get the silicone to adhere you would have to stop the leak. This means you would have to drain the tank until there is no water to leak. A better approach would be to shut off the water, drain the tank, unscrew the water line connector, then unscrew the fill valve plastic nut. Then you can remove the fill valve. The problem is that it's tank seal/gasket is leaking. You need a new one, or a new fill valve (it comes with a new seal). Fill valves are cheap, so you're probably better off getting a new one.

you could always cut a small piece of urethane or silicone foam (closed cell only) and glue it into the gap on the static side of the opening. it would act as a live seal that could adapt to the gap or move as the door rotates.

you might want to take some time to really clean the existing sealant/caulk out from all over and redo it. also do the floor to shower pan joint. use a good solvent to clean everything up and bleach all surfaces prior to re-caulking. you have so much filth and mould there that new sealants wont stick. a properly sealed shower assembly wouldnt let water into your ceiling below even if the door seals are not working.