After rebuilding the heater circulating pump, I discovered a slow leak in the fitting where the hot water enters the tub, some sort of green epoxi holds the spud into the elbow. How do I fix?

Also, after fixing the small pump, I filled and ran the large pump several minutes to circulate and purge air, but must not have been enough, soon after I truned on the heat, the GFI tripped as it now does whenever I turn on the heat, must have had a bubble. Anyhow, what kind of imersion heater do I need to replace this one?