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Do you have any clearer pictures or ones from a different angle? From this picture it looks like normal coloration change, but more in depth photos can help us to better analyze!
Plecos will lighten/darken to their surroundings to blend in, that pic looks like he's half in a shaded area half in a lighted area, a better pic would be great but I know Plecos hide and it's hard lol
I did try to get a better pic last night but he played hide and seek. He isn't half in shade tho... he really has gone half white half dark. It doesn't matter if your black or white!! Lol
If he shows no other symptoms and your parameters are fine, I wouldn't worry over it. Fish change colorations or develop new over time, it's a perfectly normal occurrence