Exponential/recursive smoothing

I need to estimate power of signal over time using exponential or recursive smoothing. I got this formula

Px[n] = αPx[n-1] + (1-α)x[n]^2

and the signal is

Fs = 8000;                 
dt = 1/Fs;                   
StopTime = 5;          
t = (0:dt:StopTime-dt)';
L=length(t);
noise=1*randn(L,1);

for alpha=0.999 power signal should look like this

How can I do it?