Lights on my pool weren't working - found that GFCI wouldn't reset. Replaced the GFCI and the old twitches for good measure. Spa light now works great, as does a socket in the backyard. But, when I turn on the switch to the main pool light, the GFCI trips.

When I pulled the main light out, and disassembled - a sucking sound occurred (indicated an air seal was still in tact?)
The inside of the light was moist, but no standing water.
I noticed some corrosion, I sprayed in some electrical connector cleaner from a home store, and then some club soda and scrubbed with a toothbrush. The recepticle looks a lot cleaner, but no improvement - still trips the GFCI - even without the light bulb in.
At the junction box, I disconnected all the wires - with a volt meter I couldn't detect any short between the lines.
I re-connected the white and black lines and the light turned on without issue.
With the switch off, I connected the green lines - GFCI tripped again.

Is that green line important?

At the recommendation of my pool guy, I'm having the light fixture sit out overnight and see if it drys out.