How to detect non magnetic material?
If the tube is plastic you could arrange plates of a capacitor around the periphery of the tube - under normal circumstances the dielectric of the cap is air but as the tennis ball passes, it is likely that the permittivity of the material of the ball will increase the capacitance - it might increase the capacitance by 30% if care is taken.

Use a 1 MHz oscillator with the capacitor as part of the tuning - probably an LC resonant tank type oscillator would work just fine - count the oscillation cycles over a short duration - as the ball is passing, the count should drop measurably.