Noise figure in the stop band of a filter

Noise figure for a passive is always equal to the attenuation. This is also the case for the filter in its stop band.
Usually it is not relevant.

Mazz

Yes, agree. Filters do not have "noise figures" but instead have insertion loss. So if you have a filter driving a low noise amp, the noise figure of the total is the (amp nf) + (filter insertion loss).

But, the noise figure of the amp may degrade if you are driving it with a filter who's impedance is very far from 50 ohms, as it would be in its stopband.