pool heater pump electric wiring
...and I have an 230v nearby oultet which has 2 wires black + white and the ground , this outlet is plugged to 50A breaker in the
eletrical switchboard, the wall outlet model is NEMA 6-15 250v

A NEMA 6-15 receptacle is only rated for 15A, so if it is connected to a 50A breaker, that is dead wrong and an immediate fire hazard.
If the conductors are rated for 50A (6AWG if you are in North America), then there may be something usable here, but not the way it is. It would be difficult however to have connected #6 wire to a NEMA 6-15 receptacle, so it's not likely. If you can, take a picture of the outlet with the wires attached to it (turn off the power before removing it). IF it is #6 wire somehow, you may need special help in terminating it to a 20A receptacle. Report back what you find.
For a 20A circuit, you need #12 wire at a minimum, maybe #10 of the distance from the panel to the outlet is long. If you have that now, then you only need to change the outlet to a NEMA 6-20
All that said, I see the JA50 pump as being rated for 2A, not 20A. So I'm not sure where you are getting that info from. Maybe it says 20A MAXIMUM, but that would not preclude it from operating fine on a 15A circuit.If that's the case, your existing receptacle is fine, but the 50A circuit breaker is still not.
But also, is this a hot tub and if so, does it have electric heating? If that's the case, you likely DO need a 50A circuit to power the heaters + the pump. If it's gas heat though, then you only need the pump power.