Arduino project LCD displays gibberish when water pump is activated?
It is hard to answer without schematics. I suspect that your water pomp is a AC universal motor and it is turned on with some kind of a relay or triac. I've seen such issues with lcd when electric motor is turned on without zero cross detector. Electric motor generates high emi when turned on with random phase and this emi corrupts lcd screen.

I'd try to add zero cross detector, maybe optotriac with zero cross detection? Like moc306x.