The function f(x)=log5^x is decreasing.

I am assuming that your function is: 
f(x) = log(0.5x). 

If that's the case, then no because: 
f'(x) = 0.5/[0.5x*ln(10)] = 1/[x*ln(10)]... 

...which is positive for all x > 0 where f(x) is defined; this implies that f(x) is a monotonically increasing function. 

I hope this helps!