PlanetCatfish.com

what does it mean when your pleco has some blotches of white/light brown color mixed with dark brown?

That's the "stress pattern" for most brown pleco's. Usually means that they are "stressed", for example if you scare them out of their hiding place into the open.

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Mats

Sometimes these patterns are also seen during the night, if you have "nightlights." This will help you make that determination.

actually yeh, yesterday night before i slept, i wanted to see if my pleco wat eating, so i used a flashlight to spot on it, is that why? will he be normal again?

It's perfectly normal. Since you didn't give the context, I assumed (stupidly) that this was during "lights on". Yes, they do have a different night-time pattern, which is lighter stripes.

Bronzefry has a picture in her album of her (late) Chaetostoma in night colour, which is not only striped, but turned green from a base colour of brown in "day-colour".

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Mats

It's a normal part of being a Pl*co. As the stress lessens, the coloration returns to normal. The water conditions, food, tankmates or any number of reasons can contribute to stress. Pl*cos are good at telling you when they are stressed. Stress does need to be remedied. It can lead to problems. Here's a small example of stress coloration:
Stressed L-147:

Non-Stressed L-147:


Stressed coloration seems to be more yellow and grey. The non-stress coloration appears to be more orange and black. This is the first ever glimpse of tummy!

If you read the "sticky note" at the top of the forum and go through those items and determine what may be causing the stress.