Hello @Bangkim and welcome to the forum!
Does the dot still show up if you perform the test for Display in Diagnostics? If you have not tried this, go to Settings > About phone > Diagnostics > Test > Display. Does the dot still appear?
You can also take a screenshot (Hold power button > Take screenshot) and open the screenshot on another device. If the white dot does not appear on the other device this is most likely hardware related and you will unfortunately have to send your device to service. Contact your Local support in order to have your device sent to and examined by an authorized service center.
