The bolt in the center of the picture is to attach a ground on the inside and bonding on the outside. This nut and bolt has come loose, so I was going to Teflon tape the thread to prevent a leak from happening in the back of the niche.
There is a product called Liquid Electrical Tape. It is waterproof once dried. Works wonders on those hard to seal connections. Can buy at Lowe's or Home Depot. I got mine at Sears Hardware before they closed the store. Very handy to have.