This is why it is a bad idea to make a queenless nuc and put it just down from the parent hive.
Cos the older bees go back to the parent hive, and tell them all where the honey is that strangely got seperated from the main hive. Their mission is to get it back.
For next time, move the queen, leave the queenless split at the original site.
For now, stop feeding syrup, or the robbing will never end. Because it's a TBH the entrance could be away from the bee cluster, and robbers just walk in. Block all those entrances and drill a hole right where the bee cluster is so there are bees right there guarding it. That hole can be plugged later if you wish.
How your feeder is set up is important. If it's one that hangs there like a frame and is open, it is impossible for the bees to defend it in a TBH, other bees just walk in and help themselves. My suggestion is a re design. Take the feeder out and make it so it has a lid across the top, and drill a hole in the side about 1/4 the way down. To feed, fill with syrup to the hole, then move some combs so the feeder can be put hard against a comb of bees. So they can go in and out the hole to get the syrup, but there are bees right there guarding the hole. Position the feeder so there are as many bees as possible between the feeder, and the entrance.
Make the syrup as thick as you can. The thinner (more water) in it, the more the bees have to spread it around into less guarded areas when storing it to get the same quantity of sugar.
So in summary. First thing is to re hash the entrance so the bees have a strong force guarding it. Second thing is to stop feeding. Third thing is to re do the feeder into something it is possible for the bees to protect.
Wait till the bees have no stores left in the combs, then in the evening give them just a small amount, a one day supply, in the feeder, and repeat daily. With virtually no stored syrup the robbing interest will decline over a few days. Just do not overfeed or that will screw everything up.
Once robbing has stopped, check if the queen cells hatched. Queen cells often fail to hatch in hives that have been robbed because the messed up bees could not maintain the proper temperature. If they didn't hatch give the hive a new comb with eggs.