Toying with the idea that every so often a little slime (or "slime baby" I guess, haha) will fall off Goo's arm or something. Slime people reproduce asexually and are instinctually opportunistic (like a bird who lays their egg in another bird species' nest and lets them raise their offspring). The slime babies will wander off and find a family to raise them and take on the characteristics of said family. If they're raised by birds, they'll be bird slimes. If they're raised by rats, they'll be rat slimes. Goo was probably raised by humans or human-like creatures which is why he's a humanoid slime.
It's pretty much unheard of for slimes to raise other slimes and the concept is totally foreign to Goo. Cameron has had to learn to adjust to all the strange things in the magical world and come to terms with magical beings thinking he's the "weird" one, haha.
Hope you enjoy my weird lore, haha! XD