From The Hollywood Reporter :
Wearing a green tie-dye T-shirt and a Philadelphia Flyers cap, Bieber was first introduced to Phish bassist Mike Gordon, then guitarist, frontman and Kanter's personal idol Trey Anastasio. According to photographer Jeff Kravitz, they found common ground in talking music, of course, and also in singing the praises of Phish's revered lighting director Chris Kuroda, who will be designing Bieber's next tour. The couple also posed for pictures backstage with various members of the Phish extended family -- and their children, naturally (see photo above, taken by longtime Phish road manager Brad Sands).
For the second set, Bieber and Gomez really got into the (Weekapaug) groove, hurling glowsticks from a virtual arsenal they had stashed at the soundboard riser. Traditionally, a glowstick war breaks out at every Phish show. This one took place during the first two songs, "Rock & Roll" and "Ghost," according to witnesses who noted, "they were having a blast."