Based on your description, that sounds like normal baby gunk (think of it as "afterbirth"), and not actual yolk. An unabsorbed yolk will appear as a yellow bulb protruding from the duck's belly, completely attached to the baby as though it has some horrendous growth on its belly. If you're just seeing yellow and bloody gunk in the bottom of the shell, that is normal, even if it's attached to the baby by a cord (the umbilical cord--this usually falls off on its own, but it's not uncommon for it to remain attached occasionally--you can later trim it if necessary, or it may still detach itself). Either way, the best thing is just to let it rest and build strength. 99 degrees is perfect. If it's the only one left, you can let he humidity drop to aid in drying, but if there are others trying to hatch OR if you think it really is unabsorbed yolk you're seeing, then keep the humidity up.