What is this white, pasty stuff growing in my outside electrical outlet? It has shorted our the circuit
It's mold, or fungus, or slime-mold. Exactly what doesn't matter.
It's an indication that the outlet is getting wet. You need to solve that.
An "In-use" cover is one way to help that along, but this also eyeballs as a half-donkey-ed "outside outlet" with a non-weatherproof box in the wall, and who-knows what as a cover plate. Can't imagine it sealed too well against raw stucco, but that's how it appears to have been done, just as if it was an inside outlet.
A 3/4-donkey-ed approach would be to put on an in-use cover with a donkey-load of sealant to attempt to seal it against the stucco.
For the entire donkey, you'd want to replace that box with a weatherproof box and an in-use cover.
I'm pretty sure that's some sort of caulk or other glop that's been spread on the outlet. The coloring is far too uniform to be some sort of mold, and the way it's spread over that top outlet would not result from mold growth.
Of course, this doesn't change what is needed to fix things. The outlet needs to be replaced, and a decent cover installed. If wind-blown rain is apt to be a factor the box should be replaced with one that's weatherproof.
I suspect that the installers might have used some sort of interior product to seal the outlet to whatever cover it had previously. Usually caulk or drywall. Exterior products are waterproof and will not promote mold.
I agree with Ecnerwal about water intrusion, and I suspect the source was whatever they put on for a face plate. See the dribble of rust in the lower left corner of the box? That screams that rain is hitting your stucco, running down it, and then getting into the box itself. I have a couple of suggestions here
- Upgrade to a GFCI. Be sure to buy an outdoor model (with the doors on the front).
- Buy an exterior extension ring and exterior caulk it up good (top and sides only to allow the box to breathe). Yes, your box will protrude more, but the new setup will better resist the water intrusion.
If you do that, any exterior cover should suffice.