Maybe it's just colostrum baby poop...probably it's just colostrum baby poop...
At this point, with the calf nursing his mother well, I don't know that I'd worry about it...at least not at this time. Chances are you won't see it like that again.
Josh, what color is your soil...could the calf have been laying down in something like red clay type soil and pooped and...nah, that's a stretch.
Keep an eye on him, but I really don't think it's anything to worry about. However, my experience is in bottle baby calves, so others might very well have a much different opinion.
Alice