not black and white. Rather depends how big it is, and how the windings are insulated. on a large machine with possible condensation on internal bare terminls it may be nothing to worry about. If it is supposed to be a small sealed unit, then there is trouble ahead.
Also what would happen if it failed - it may be worth replacing it early rather than waiting if it is mission critical. If no one will care that much, it may be worth letting it run to failure.
Do you have known good one (or indeed results from previous tests last time) to compare - if it is a small change, then not as serious as if it was ten times worse say...
probably slowly getting damp in past the shaft seals. It is not going to trip any RCD or do anything nasty at 1M ohm, but the fact it is getting worse is not a good sign - I'd suggest it can stay in service for now but the users should arrange to have a spare to hand, as one day it will probably go off and stay off !
Presumably when it fails it is not a repairable unit, so there is no option to have it serviced early.
