Yeah I don't think thats normal, I just changed the fan belt cos' the air con clutch was sqeaking so I bought a shorter belt to skip it. But I didn't notice any extension to the alternator
But if it's causing no problems and the belt is tight, then I won't be too worried.
I'm just a bit confused, in which way does the alt move around, cos' the tensioner should just move it up and down, pivoting from the bracket. If it's moving side 2 side then I'd fix it.