Justin Bieber -- What's In the Water Bottle?

While Justin Bieber strapped on his best hip-hop inspired accessories to hang with rap royalty Wednesday night -- one particular piece of flare seems to stick out ... the mysterious green inside his water bottle.

After posting pics of the underage Bieb's keeping his cool while hanging around dozens of bottles of booze at Diddy's shin-dig at Vanquish nightclub in Atlanta -- we noticed Justin's water bottle had something inside other than H20.

The rest of the A-list entourage enjoyed a neon yellow cocktail that could have easily been poured into Justin's clear water bottle.

Unless you're at the Sochi Olympics -- this water doesn't look normal.  So what is it?