It's on his right nipple. Scroll down in this post on Evan Namerow's blog Dancing Perfectly Free to see what it looks like. I think it looks fine, but your mileage may vary. (I live where the East Village meets Union Square. In my neighborhood, Hall's tattoo would be considered a nice start.)
Now that I finally saw the tattoo - oh yes - he could EASILY dance the role of Apollo. The top of his costume - the sash - would easily cover that up. Add a little flesh colored make-up for safe guarding and you wouldn't even notice it was there. And even if NYCB was still performing the "Birth of Apollo" prologue the design of the tattoo is actually appropriate. Not only is it tastefully and elegantly design it depicts the sun and of course Apollo is the Sun God. Yes he has my permission to dance the role. LOL.