So, does anyone know when it will be open for the public to purchase? I might sell mine beforehand.

Well, maybe never. Since is a non tradable item.
Like some collectors cards and etc.

It will be tradable, but not anytime soon.

What does the green top-hat indicate?

It indicates self worship, seeing oneself above the others
Green normally associated with nature, the natural world
While the top hat itself signifies the cruel Business world
And the battle to the top.

Those green hats tells us that about yous.

Where did the green hats come from and how were they obtained? Was it an event?

FIsh gave some people these green tophats on their avatars. After that, some (or just one) hat/hats were auctioned off. Now everybody who has the green tophats on their avatar has been given the green hat item.

No, but there are upcoming events where you can get green hats.

Fish why y u do dis the 'F' doesn't look like a green hat on your name :c Change the font of the F to make it look like a Green Hat

any blasphemers that try to sell their green hats should be perma'd