Can I use a 50 amp GFCI breaker on 15 amp circuit?
Yes, you can use this.

GFCI breakers combine two different functions into one device.  The circuit breaker function of the device is rated for 50 amps.  However, because your 15 amp breaker is upstream from the GFCI, your circuit is protected at 15 amps, and will trip at 15 amps if there is a fault either before or after your GFCI.

The GFCI function on the GFCI breaker is not related to the 50 amp rating.  This will trip if there is a ground fault after the GFCI breaker.

As long as you size your main breaker correctly, you can use this GFCI breaker at the local disconnect for the spa.

See this article for more details on wiring a spa: SpaDepot: Wiring a Hot Tub