I like the brownii (not braunii) Selaginella. I have it in my terrarium now, and whenever anyone looks at it, they say how much they like the "moss." It's a nice, bright green. Mine is about a half an inch high at this point, I bought it about a month ago.

Hi
One that I've found that consistantly has that growth habit is S. martinsii. It will choose to grow down rather than up even when it has something to climb on,
I find all the selaginella family extremely variable,. growth habit dictated by humidity and temps and of course intensity of light.. Growing in different methods ,the plants look like entirely different species by habit alone.
Martinsii also appreciates ,stable warm humid conditions on a year around basis .
gary

Selag that is trying to grow up generally seems to do so if light is a bit too low. Pressing it back down to the substrate -- perhaps even setting smoething on it temporarily will help "guide" it back down as it anchors that portion of itself to the substrate.