What's the difference between Halium and Mer? They both seem to be similar projects.

Packaging. If I understand correctly, Halium's goals are simpler in just doing enough to bootstrap the libhybris layer.

This allows projects such as Mer, Plasma Mobile, LuneOS, UBports and AsteroidOS etc to pool efforts without any distro-specific assumptions.

Currently it's the Nexus 5.