If it were a farm-type game, the white/gray stuff could be the wool of a sheep, or a white gravel driveway.

Like somebody else also said, it could be the pebbles from the bottom of the tank, but what puzzles me is why would there be grass in a fish tank as well?

On the other hand, the white could just be a border...

And like Kayy said about it being based on a beach, I am also thinking that the white could be turtle eggs! Or maybe even a dinosaur egg, judging by the rough texture.