If you really dislike it, you can use a toothbrush or sponge to scrub it off.I'm not a big fan of my hardscape being green, but the shrimp love it and so...
I'm of the opinion that it looks good in a shrimp tank, though, and ends up being okay if it's just on a few areas of hardscape like that. Never lasts very long in any my shrimp tanks, so it may be something that disappears relatively quickly for you.
What's your lighting situation? Cutting that down a bit may help eliminate it.