Police now believe the powdered drug they confiscated at Justin Bieber's house -- allegedly belonging to Lil Za -- may NOT be cocaine ... they think it's probably Molly ... a form of Ecstasy.
Law enforcement sources tell us, they're testing the drug right now and should have results shortly.
In addition to the white stuff, we're told police also found Xanax ... which they believe belonged to Za as well.
If Za was in possession of Molly (MDMA), less is on the line than if it were cocaine.
Cocaine could land him in prison for 3 years, but punishment for MDMA is much less, about 1 year.
As we reported, Za was arrested this morning inside Bieber's home while police were executing a felony search warrant in connection with Bieber's egg attack on his neighbor.