What is this coating?
Looks like anti-corrosion gel to me.
At first I wanted to say anti-oxidation compound but the more I look at it it appears that someone shot a resin into the ports in order to not give the ability to use the device for it’s original intended function. it looks like a 2-pole being modified as only a 1-pole device…thus they blocked up the terminal ports.
While not normally advised…I would have poked it for hardness with a nonconductive probe to see if it was indeed a resin. But thats what It looks like to me…just trying to make you use it as a single pole only device.
That’s factory installed resin of some sort used on older ITE GFCI breakers. I have a box of them laying around somewhere. 
They were GFCI. There were two of these breakers. The application style of the resin on both breakers was consistent.
Agree. Nothing to worry about.
I dug one out of a box in my garage: 
https://lh4.googleusercontent.com/eJzjkdcnL6wGhcaRFyl3rjp-pbuab7vFhB5qu4dvt88=w446-h786-no
https://lh6.googleusercontent.com/T-PSxHdfT1bt1N-a9rFeiyt6ga6es5foKm0-2TBW5gI=w1161-h786-no
no guts no glory. Wet the tip of your finger and try and dig it out! :mrgreen:
lol…guess I am not old school enough…lol…
It’s a factory installed epoxy resin, it prevent use of the connection.