This bug does not affect the gamplay but its weird nonetheless, in version 1.16.1 i saw on one of my worlds a white enderman, i tryed to summon another enderman to make sure that it isnt something that was added in the game as an easteregg but they were bouth white, not to sound racist. Here is a picture i took of it:
You are using optifine. This is not in vanilla. Report it somewhere else.