Talked to every race but the insects and they all have blurbs about not knowing what to do, but none of them give any indication about the crab guy.
At this point you can try updating to version 1.0.13c. It just popped up in the offline installer section. It's meant to fix some progression issues like that.
Just make sure you're talking to the right crab first: vv
The Ulooquo (squid) didn't know either, but had a dialog option for that quest. They tell you to talk to the Gortanu (crab) which I suspect unlocks the dialog option for the Gortanu. I did it in this order at least. HOWEVER, ONLY AFTER you talk to the REGULAR Gortanu in the Gortanu world (top right on the map), you can receive the message from the Hermit (crab guy with the prophecy - for me he's left on the map in the Tyn Coeden / tree world).
Hmmm I didn't talk to the colonist crab guys... though you have the option to never reactivate any of them, so you'd think that would not be connected to end game content.
Edit: Went back to the colonist crab guys and they had nothing more to say; planet is not interactable anymore.
I'm on 1.13c and nothing has changed.
Only odd thing is that the patch notes are greyed out in Galaxy; I don't think the devs uploaded patch notes. :-o
Edit 2: Oh wow, I didn't even remember those machine loving spaceship crabs existed!
I only found out when I found a redit thread after searching their name.
it seems like a monumentally bad design to have the end game be dependant on a seldom encountered possible-enemy when you've already found the prophet crab on this planet! :-(
Thanks for your help!