When Arcane leaves New Deva for the Galactic Time Guard

When he leaves New Deva he believes (erroneously so at that moment) that he can never return. He hasn't figured out yet that he CAN return before he was about 27 and meet his young self and parents and others and that this could work (which he eventually does) and becomes known as "Uncle Future" to his young self "Arcane". But, in life you only know what you know when you know it.

And that is true of all of us. Would we have done anything we have done in our lives if we knew then what we know now?

That's why in time lines: "You only know what you know when you know it." (whenever that is in time and space or on any time line) is a very important factor in anything that happens anywhere in any time or time line.

You might say "Why doesn't young Arcane recognize his future self?

Because Old Arcane has had extensive plastic surgery. He no longer has a 9 inch tongue that was useful underwater as a fish lure for an amphibious human race like the one that evolved on New Deva. He no longer has bright golden irises now (because he wears contact lenses to cover them up).
He no longer has purple skin or webbed fingers and toes or elongated fingers with webbing for swimming faster either.

So, for all intents and purposes Old Arcane doesn't look like young Arcane at all because of extensive Galactic Time Guard modifications for long term living and fitting into Earth cultures from 1987 to 3000 AD specifically but for all other times of earth too past, present and future.

So, when he returns he is a relative who has been altered for the Galactic Time Guard which is a very respectable and rich kind of trade for someone who was originally from New Deva. So, Arcane only lets his parents know who he is in these situations except for people he brings from earth over time to visit New Deva 1,000,000 years into the future.