Worms :-) Those are worm castings, aka worm poop. A very nutrient-rich, organic byproduct of the natural soil biology. They are a good thing......just rake to spread more evenly.

Thank you so much for the quick reply gardengal! I would have never guessed worms! When do you think would be a good time to rake without damaging grass? The snow just finished melting off, and the soil & grass are very wet.

Just a few days without any rainfall should work, The castings break up very easily.