Usually, hot n-well in nwell that is not connected to VDD.
You should check that the Nwells are connected correctly to VDD. If you want to connect the nwell to different potential,I think that you can ignore the warning.
"hot nwell" is the well that's left floating or say not connected to vdd. this could result in a forward biased pepi-nwell junction, which in trun could lead to latch-up.