It reads that the possessing initiate can only target someone who is astrally active, but projecting mages/shamans do not count as they have no body. The latter is a no-brainer. But what constitutes being astrally active exactly?

I'm curious as I'm writing a run now that involves a toxic free spirit with the possession power, whom I intend to use either on a PC or by being called into the body of the toxic shaman. I just want to make sure I'm down with all of the rules. Thanks.

Dual natured: always astrally active.
Astrally percieving: temporarily astrally active
Astrally projecting: astrally active but has special rules on how to possess

edit: too slooooooow

Is casting a spell astrally active? Also, what about active metamagic not masked?

Spellcasting only requires astral activity when casting along the astral plane. As a concept, metamagics don't require astral activity, but some may specify it in their descriptions (psychometry I think is one). Masking does not change your level of astral activity, it can just make you look less astrally active than you are (among its various uses).