Black Flag Founder Greg Ginn -- Creepy Concert Footage Fuels Custody Battle (VIDEO)

Black Flag guitarist Greg Ginn is not to be trusted with his 2 daughters ... which his ex-wife says is obvious to anyone who watches this video of him perving out with a young chick on stage.

Ginn performed with Black Flag in July at a Butte, MT music festival. A young woman jumped on stage and gotta say ... what he does with her is pretty shocking.

The guy who shot the video says it was so creepy ... he contacted Ginn's ex-wife because he feared for the safety of the couple's daughters, ages 10 and 7.

Marina Ginn says her ex has become increasingly erratic, and in court docs she accuses him of going off the rails with drugs around the kids. She says he neglects to feed them when they're with him and disciplines them by throwing water in their faces.

Marina says they currently have joint custody ... she now wants sole custody.