The NHL explains the call...

via NHL.com,

At 17:14 of the second period in the Lightning/Canadiens game, the Situation Room observed the puck cross the goal line but play continued until 17:50 when a review was initiated. The referee informed the Situation Room that he observed incidental contact in the crease by Lightning forward Tyler Johnson on goaltender Carey Price, if in fact the puck crossed the goal line. Video review confirmed that the puck crossed the goal line. According to Rule 78.5 "Apparent goals shall be disallowed by the Referee when an attacking player has interfered with a goalkeeper in his goal crease." This is not a reviewable play therefore the referee's decision on the ice stands, no goal Tampa Bay. The clock was reset to 17:14.

The NHL has not posted a video of the play but SomeHockeyVideos did at YouTube. The video is from RDS and of course in French.