Xellos, on his end, will notice Nico for two reasons. First - he's able to sense other supernatural beings and even if he won't realize the details, he'll know Nico is more than human. Second and most important - Xellos feeds on miasma and negative feelings. The bad vibes that make normal people nervous? Xellos will be all over that. Combine it with all the negative feelings Nico has and Xellos will home on him like on a beacon. Chances are he will do his best to poke him so he can increase that delicious bad juju.