Tried charging a penny/dime battery. Yellow goo bubbled. What did I create?
Hahaha, I had to translate for myself what a penny and what a dime is. Always the same for me; Fahrenheit, miles, ...

A penny is a copper-plated zinc coin. A dime is a nickel-plated copper coin. Knowing this, I would assume that the yellow color arose from the nickel content. If it would be blue, it would have more likely been copper.
The goo. Such a structure often occurs with proteins, polymers, and similar compounds. It's most likely that the bacterial farm on the coins you used, lead to the formation of this goo. It's impressive how many bacteria and what thick biofilms you can find on a coin. Additionally, your saliva also contains a multitude of living organisms and their remains.

Therefore, you should definitely reproduce this experiment! Clean the coins beforehand using ethanol, acetone, and water while scrubbing. Polish them with a linen cloth and not with sand paper. Otherwise, you would damage the plated metal film and surface the underlying metal (makes things now complicated). And don't spit in it yet! :-)