I never played an island aumaua before but wanted one for a fast moving mage slayer. On the first island my MC is being affected by the slog zone caused by the pool. Is this just an issue on the first island or will all slog zones affect my MC?

It's not a real immunity - it works only during combat (like the description says).

historical lesson - this "only in combat" effect was introduced in an early patch because apparently it was considered an issue that it was hard to keep parties including island aumaua together, because the island aumaua would get way ahead of everyone else.

i don't really agree with that decision, because a lot of people were confused by it (and still are, apparently), but that's why it's the way it is.