Lil Za thinks he was arrested during the raid at Justin Bieber's house because he was the only black guy in sight.
Za -- who was arrested for felony drug possession Tuesday -- just tweeted, "Some stuff I've done has been wrong yes. but look at the position I'm in and the color of my skin. Cocaine? Really?"
Truth be told ... cops think it's Molly ... a form of Ecstasy.
It's unclear if Za is saying he was singled out because he's black or that cocaine is a "white guy's" drug.
Za was also arrested for beating a jail phone to death and charged with felony vandalism.