To fix the water damage and shower head's decreased flow, thoroughly clean the tray out. Eliminate all the remains of the mineral deposits on the shower tray. Hard water usually blocks the pipe stem and the shower head. Thus, leading to a leaking shower or damaging the pressure and flow of water.
Pull it out and replace it with one from the hardware store. After the soaking, free the holes with a pin or toothpick that fits, then brush the faceplate clean with a stiff brush. Any debris that falls into the head should flush out once you reinstall the head and turn on the hot water. BicFnco.