Can I vent a portable air conditioner inside?
It's possible but it will be creating a situation where your a/c is fighting against itself, because you'll simply be redistributing the heat inside your house, instead of putting it on the outside.

Why?  Because the warm air will creep back into the room you were cooling, so the a/c will be acting like a big do-nothing machine - to a degree anyway.

Mold isn't going to be too concerning, however, because you won't be taking moisture out of the one room and putting it into the other - the moisture in the air in the warm room may increase a little (because warm air can hold more water vapor than cold air, so if moisture is there, it'll get picked up) but not to the degree it would if you were, say, venting a dryer into that room, where you'd be introducing lots of NEW moisture. 

All that said - I do not recommend this idea.  Better to create a vent/opening (properly, of course, but that's a different question) in the outside wall to allow the warm air to go where it is supposed to.