Yep, grub or larva. Have no idea what kind, but they'll definitely do damage, whatever kind. The nice thing is, that one probably just dropped in. If you're not seeing a hundred of them, it's doubtful that an egg sack of them hatched out. Even so, as Traci suggested, either submerge the pot for an hour or two or repot in fresh medium after getting all the old stuff off of the roots.