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This appears to me to be damage caused by low angle sunlight reflected and focused by a nearby surface. There have been other message threads in this forum related to similar damage to the "sail panel" plastic trim between the front and rear side windows.
Fortunately, this "mirror base" plastic piece is available separately and not exorbitantly priced (by Lexus standards). After a shopping cart incident, I purchased this part from an Ebay seller for less than $40 and replaced it myself. The replacement piece fit perfectly and had nicer surface finish than the original factory part. It was not an easy job and requires some patience, but a lot less expensive than the cost of a complete new OEM mirror (several hundred $$).