Just imagine me hopping up and down in the background waving my hand all 'Pick me! Pick me!!!!!' haha. Nico's whole power set is based on him being a son of Hades so he dealt with dead people all the time back home but he hasn't been able to do much with that here because people get resurrected instead of staying dead dead lol. Nico would absolutely be able to talk and chat with Spirit Atem and even interact with him like a normal person. He's grabbed ghosts and physically dragged them away in canon before, hahaha.

And feel free to have Atem do ghosty things around Mokuba, he wouldn't be able to see him but I am all up for Mokuba getting freaked out a bit until he puts together what's going on haha.