It is a cotton topped tamarin x common marmoset, it was a total accident which happened about 2 years ago now and many people i have spoken too believed it couldn't happen. Apparently only a handful of tamarin x marmoset hybrids have happened world wide so it does happen although very rarely, which is a good thing!
Imagine how many marmosets and tamarins are kept in rainforest type mixed species exhibits throughout the world...