That whole thing was done wrong.
You would be way better off taking the time to fix it right.
This time use a nail on encloshure not a cheap thin glue on one.
It's attached directly to the studs and is at least three times thicker.
That enclosure never should have been installed sitting above the tiles.
An encloshure needs to be sitting just above the lip on the back of the tub to prevent leaks.