My daughter has a three piece tub/shower. Along the outside edge, where the top and bottom pieces meet there appears to be some water damage to the drywall. This damage appears to form a half circle in the drywall with the edge of the tub/shower seam at the center. Many months ago I tried to caulk between the upper and lower pieces to see if that would help, but the bathroom caulk I used didnt hold. Any ideas on how to fix this. Photos attached. Thanks.