As people have mentioned above, the best thing you can do is to report the player.
Even if staff were to be online on the server, they might not only want to go around lobbies, looking for people with inappropriate skins. Even if it's a staff member's job to punish rulebreakers and keep the server nice and friendly, they are also considered as normal players and have to be able to play the game as anyone else.
With that say, either way, even if they are just chilling in a lobby or playing a game, you as a player need to help them out to cover a bigger part of the server. Player reports are pretty important and just generally help the staff team and the server a lot.