Hmm, that's interesting. I see what you mean about the big legs.
I don't know, so I'll fall back on the usual advice: keep an eye on how it grows, and hope it becomes clear in time.
CX stands for Cornish Cross.
They are hybrid meat chickens, that grow white feathers and have very thick legs. They grow very quickly, so by the time they are 2 months old they are heavier than most adult chickens of other breeds.
Cornish Cross chickens are usually butchered around age 2 months, because they usually develop health problems from growing so fast.
The name is because someone crossed a Cornish chicken (one breed) with another breed, about a hundred years ago, and found their their chicks grew bigger and faster with more meat than any other kind of chicken they had been raising up until that time. Over time, they selectively bred chickens to produce a cross like that, and they no longer have actual purebred Cornish involved, or any other pure breed of chicken in the mix. But the name stuck.
Purebred Cornish chickens do have a meaty breast and thick legs, but they do not have the explosive growth of the Cornish Cross hybrids (CX). They are also relatively rare. They come in several colors, including Buff that can look a lot like the chick you have.
Your buff chick is definitely not a Cornish Cross (CX), because yours has buff colored feathers, and Cornish Cross are always white.
Some hatcheries have been developing other meat chickens lately, with colored feathers and an in-between growth rate. Some of them have thick legs, so there is a chance your buff chick is one of them, but I can't tell for sure.
Can anyone tell me what they think this gray black and white chick's breed is?
Where did you get the chicks? Sometimes that helps with figuring out what breeds they might be.