Hello, so, I decided to discover Ultima Underworld.
And so, while it is said that in zones with an ankh, npc's won't be hostile, when I enter the green goblins non-hostile zone, I'm attacked by some green goblins (but some of them aren't agressive). Any idea why? is that because I spoke to grey goblins first?

None of the non-hostiles care whether you speak with any of the other non-hostiles. So you are fine speaking to both greens and greys.

So you talked to the guard, had him open the portcullis, and went inside. Where in the area did the goblins start attacking you? Did you open a locked door that had a plaque next to it saying something like "Storage Room"?

They won't attack you just for having spoken to the grey goblins. The only reasons they would attack you is if you attacked one of them, if they notice you stealing from them or if you break in the door with the "Keep out" sign. If you have done none of these things it might be a rare bug.

The goblins attacking you; are they wearing red or just brown leather and metal? It is unlikely, if not impossible depending on the AI, but one or more of the always-hostile rogue goblins (who wear red) might have made it's way in there somehow.