Depending on where you got your plants, i.e. if you bought them from a nursery, then that's probably where the slugs came from. Plants are good places for bugs to hide. Did you clean the plants prior to introducing them into the terrarium (meaning did you give the plants a through rinse and change the potting soil)? It's always a smart idea to do this. On the basis of feeding your frog(s) the slugs, I wouldn't advise it. You don't know where the slugs have been, and if they were from a nursery, they might have fertilizers on them or some other harmful substance. I would just get rid of them