Peat moss (I think that's what "sphagnum" is) has been used for centuries (probably millennia, actually) as a wound dressing. It draws a lot of fluid out of wounds. Has been replaced by sterile cotton wrap and other absorbent materials. Best, I'm sure, not to apply directly to the wound, but as a middle bandage layer (between a wicking internal layer and a protective and/or moisture resistant outer layer).
Don't know of any inherently antimicrobial properties, though it wouldn't surprise me if it had some.
Here's some stuff about it and how it's been prepared in the recent past. http://www.botanical.com/botanical/mgmh/m/mossph54.html#pre