Can anyone tell me what the green numbers mean while you're in combat?

it means your poisened and you should take anti poisen pos or eat to keep up with it

Well judging from the fact that this is in the General forum (which is for f2p as well as p2p) I'm guessing there's a chance the poster is F2P. If that's the case, the person you saw getting hit with green "splashes" is a member that foolishly switched to an F2P server in an attempt to get away from the effects of the poison, when in reality they put themselves in a position where they're still poisoned but cannot drink an anti-poison potion to cure themselves.

With certain herblore levels, you can poison certain weps, mostly range weps, but also daggers and spears, regular hits 4, plus hits 5, and super hits 6. they are noted by (p), (+), and (s)