Dunno, hard to say without seeing it - could be moss or algae, in which case use mosskiller, could be some sort of broadleaf or evergreen weed, glyphosate might work. Or not, depends on what it is.

If its moss there's a purpose made moss killer spray but it would only be economic for a fairly small area.
Again if its a small area there was a tip on gardeners Question time some time ago - boiling water. this particular panel member said he empties his veg water onto the joints. I guess if you don't use it for stock...
Jeyes fluid if you can stand the smell.

I just use a line of salt on each joint. Only 25p a packet from supermarkets and once you put it into a salt container, you can merrily get into a pattern and cover an area quite quickly. Works for me. Don't put it near your plants though.