That'd be a Visitor-manufactured t-shirt lifting the porno cover art from Mokinox's Im Your Chair CD-R, which--when not obscured by a guitar and some squeamish TV cameras--looks like this:

Mokinox is the one-off noise project of Chris Habib, designer, filmmaker, and Sonic Youth buddy from way back when (his work is featured prominently in the traveling Sonic Youth retro Sensational Fix, currently at the Malmo Konsthall in Malmo, Sweden.) Visitor Design is his official professional outlet. And that is one gangster t-shirt to wear on national television. Not that you could really tell. Moore rocked it for SY's blindly tight version of "No Way" on Fallon's show last night, which was pretty gangster itself: