Morgan Wallen Concertgoer Says Man Peed on His Family During Baltimore Show

Talk about piss-poor concert etiquette ... a man allegedly whipped out his penis and pissed all over a family during Morgan Wallen's Baltimore show ... and now police are investigating.

Troy Grevelding tells TMZ ... he was at Saturday's concert at M&T Bank Stadium with his wife, brother and 8-year-old niece when a man directly behind them began urinating ... splashing Troy, his family and another nearby group.

Troy says he and his wife could feel the stream run down their legs ... and when he turned around, he claims the guy still had his junk out.

Event security booted the man ... but Troy says the first Maryland State Police trooper to respond stated cops couldn't arrest him because the trooper hadn't witnessed the alleged incident.

Troy says a Maryland Transportation Authority Police detective later tracked down and detained the man outside the stadium.

Video from the concert shows officers talking to Troy while explaining how he and his family can pursue charges.

Troy says MDTA Police is handling the investigation.

Troy believes the man was intoxicated and sitting in someone else's seat. He says the guy had been obnoxious earlier -- loudly talking on his phone and bumping into people -- but eventually chilled out before things took a disgusting turn.

Troy also says someone from the stadium promised to make things right ... but he has yet to hear back.

MDTA Police tells TMZ they are "aware of this incident and the video that is on social media. There is an active and open criminal investigation into this matter."