Is it ok to hit the piezos connected to analog pins while the arduino is off?

I was just wondering if it is ok because I'm afraid that the arduino might get damaged.

Thanks in advance to whoever answers.

Yes, It's OK.

Thanks.

Piezos generate small currents only, these will flow harmlessly through the input protection diodes.
Of course there are limits - higher frequency and larger piezos and more power will eventually mean
too much current, but typical small piezo disks should be fine - remember you can always put a 10k
series resistor in line if you like.