It is a stem "plant" (actually a macro-alga) that looks a bit like a compact hornwort. There are two major genera, Chara and Nitella. Chara has rosettes of simple branches extending out from the main stem, while Nitella has additional rosettes of smaller branches on its branches. I haven't had much luck with it; it tends to shatter in my tanks, but I've only tried wild-collected, not tank-propagated.