It is colloquial, but not slang, per se.

In the Southern, and some parts of the Midwest and Southwest US (Nebraska, Kansas, Texas, Louisiana, etc) it's very commonplace to hear people say "y'all" which is a contraction of "you all". You won't find it in a standard English dictionary but it is a valid albeit sometimes frowned upon expression.

So your answer is yes and no. It's certainly a real contraction to the few million people who say it, but it's not considered "proper" (whatever that means)

-Nah-