How to clear the Photos icon badge
If you haven't already, turn on the sidebar - Option-Command-S - then look towards the top and under Shared, click Activity. Likely there is some activity on a shared album or Photo Stream. It may also be someone inviting you to join a shared album or Photo Stream. (I'm not sure how that would show up in Photos, as I haven't had any invitations since Photos was released.)

Either way I'm guessing once you take a look at that area, the badge will disappear.