Looking at the surroundings, I wonder where a grass seed came from? Thinking it was in the soil that came with the shrub.
If it can't spread past the little spot where you're watering the shrub and into the surrounding ground - that looks like concrete, I'm not sure why it's a problem. Where there is nothing growing, why is anything green not welcome - even a hideous vine-grass? I'm not trying to be a smart-alec, I'm genuinely curious. Cardboard or mulch would smother it, or kitchen scraps, if sacrificing anything green in such a spot is desired.
