Lets say I have a {Merrow Reejerey} in play and cast {Phantasmal Image}, copying the {Merrow Reejerey}. Would that count as casting a merfolk spell for the ability of {Merrow Reejerey}?
When it enters it is just an illusion, not a Merfolk, so sadly no.
Well, the answer is still no, but for a different reason.
When phantasmagoric image enters, it enters as a copy (including types) plus illusion as an additional type. So, when it enters, it is, in fact, a merfolk.
However, when you CAST the spell, it is just {phantasmal image} itself on the stack, which has "illusion" as it's only creature type, so it will only be an illusion as far as "when you cast (creature type) triggers" are concerned. ETB is another story.
RESOLVED