I'm not sure if you're using the C++ sqrt function on a Windows computer, which has it's own problems, sqrt function of SF, or another OS altogether (I'd assume the first). I swore I remember there being a computational error on Windows machines when calculating square roots, but I can't seem to find any references to it online.
I finally read the issue you're having (I had assumed you were having a seg fault error or something). But if you're following the mouse around, why can't you just do sprite.move(mouse.x, mouse.y) if you're trying to have the sprite follow the mouse. Otherwise, plant some numbers in, just say the sprite is at 100,100 and you move the mouse by 10 in either direction. Does the math reflect the desired result?