hello & welcome
type of spot you are talking about may be a case of mucosal blenching, in upper dentures usually a mid-line relief is given & lack of that may cause blenching.
in long term cases this may cause ulceration or burning sensation.
stop wearing denture for 1-2 days and notice whether white spot is fading or not, if yes than visit to your dentist
for further assistance.
Rest take care of hygiene of your oral cavity as well as denture.
thanks & take care