That is from the Lithium battery in them.
I have 5 of those bark collars, it is the same as the innotek bc 50. I had one leak like that with a lithium battery. Luckily it was not that bad. I use the regular battery and never had a problem.
Good luck with your dog.
ya I used a lithium
Sent from my SAMSUNG-SGH-I847 using Tapatalk