Yes, Fm7b5 is a substitute for F#m7.
I'm not entirely convinced of this, mostly because there's an F instead of an F# in the chord.
To my uneducated ear, the Fm7b5 is functioning more like an altered B chord that it resolves into a less-altered B chord..
Since the chord is mostly minor thirds, the B-D# interval sticks out because my ear wants a strong interval.
Just my 2 cents.