f(x)=√(-x) only exists for -∞<x≤0 and over this entire interval the function is decreasing (to zero).
f(x)=(-x)½, so:
f'(x)=-½(-x)-½=-1/(2√(-x)) which is always negative (function is always decreasing).
f(x) is the upper half of a sideways parabola with vertex at the origin.