When Mavis comes back from the dead as a ghost, she says that only people with the Fairy Tail mark can see her...what about when she talks to Zeref again...can HE see her?

no, zerf can't see mavis

wewe have to watch fairy tail Zero for that answer

it's pretty clear after watching that
Mavis get's to control who sees her and obviously that's everyone related to her ( markbaring people ) and "zeref" as well

there is a connection ( clearly hinted at during the set arc ) and plotchanging in fairy tail zero

yeah, that's some news aint it, the real reason fairy tail even exists to begin with ^^

Zeref can't see Mavis, but he can sense her, to put it simply since they have history Zeref can feel Mavis's presence, wewe need to do a lot of searching to find the answer to your question, don't worry I felt the same way.