Well The Green one?

Since the four are disagreeing then 3 must be lying. Since only octopuses with 7 legs lie then there must be 7*3 = 21 legs.

That leaves the honest octopus with either 6 or 8 legs. So the total number of legs should either be 21+6= 27 legs or 21+8=29 legs.

Since no one says that they have 29 legs then only the Green octopus is saying the truth.

It seems to be simple unless I missed something in the riddle.

Well The Green one?

Since the four are disagreeing then $3$ must be lying. Since only octopuses with 7 legs lie then there must be $7\times3 = 21$ legs.

That leaves the honest octopus with either $6$ or $8$ legs. So the total number of legs should either be $21+6= 27$ legs or $21+8=29$ legs.

Since no one says that they have $29$ legs then only the Green octopus is saying the truth.

It seems to be simple unless I missed something in the riddle.