Since forming six years ago, Charleston acoustic duo FolkGrass has developed a flexible performing style that allows for a pretty eclectic mix of material. They do a little bit of everything while living up to their name.

“I think the sets probably sound fresh to a lot of people, whether it’s old friends from way back or people who’ve never heard us before,” mandolinist Aaron Firetag says of the duo’s ever-growing repertoire. “I’ve recently worked up a few new fiddle tunes, and I’m singing a little more. It’s never been just familiar standards. We’re pretty good at picking out really obscure folk or rock tunes.”

Firetag and his bandmate, singer/guitarist/harmonica player Bryan Watson, might jump into an old Chet Atkins classic like “Freight Train” before veering into a slow blues favorite by B.B. King like “The Thrill is Gone.” They can get strummy and start blowing the harmonica like Dylan or mellow out with some Old and in the Way-era mountain music.

“If you’re a young folkhead or an old Deadhead, you’ll recognize more than a few songs,” says Firetag. “At gigs like this one at the Moose, we don’t expect to get requests for ‘Rocky Top.'”

Firetag keeps busy playing mandolin with three other projects as well, including the ever-changing groove/roots combo Quasiphonics, the pop/folk quintet Lost Highway, and the wild punk-metal band Skwirl Grinda. He still loves jamming as a two-piece with Watson, who has two kids and a busy family life these days. They only plays occasionally, so they try to make each gig count.

“Each time Bryan has a child, he takes time off, but we get back into it,” says Firetag. “It’s easy for us to pick up where we leave off.”

Sonically, acoustic guitar and mandolin complement each other without getting in each other’s way. Watson’s flat-pickin’ tends to stick to straight-ahead chord progressions that imply underlying bass lines on the bigger strings. Firetag uses his mandolin as both a rhythm and lead instrument, filling in the spaces with peppery riffs and a few high-note melodies.

“I’m glad I was able to convince Bryan to play this week as FolkGrass,” says Firetag. “The last time we played at the Moose, we had a nice draw. This time, we’ll probably have people packed in the kitchen, behind the bar, on the bar [laughs]. It should a blast.”