Damnit! Every time! Our UI Dev asks us to change the API every time he wants something different!

The API is backend -> business logic. You don't change your business according to what fucking color you want on your site!

Sad part is since he's senior I don't want to tell him off

Dont worry about him being senior. If you see a problem with the process and certain decisions. Raise them in the appropriate forum. If you have retrospectives that can be a good time for that. Or if you dont have that opportunity, then raise it with your line manager. If your company isnt open enough to facilitate raising concerns even with people above you, then it doesnt sound like a good place to be.

Tell him. I always call out the bullshit everyone else does and expect the same from them.
If they can't work with criticism it's a crappy place to work

In the Dev world, "senior" doesn't mean jack. If there is a problem, you either speak up to work towards a solution, or wait around long enough to be the one to blame.

I'd tell him off.
But I'm not very nice and don't put up with idiocy.