Crabmones are a throwable object in your inventory that you can pick up from nests. They are greenish eggs in the nest and you simply need to touch one to pick it up. There will be dialogue that will tell you this when you first come across them.
Select it from your wheel using
and throw at a crab. It works like bait and they will eat it and get a greenish aura around them and will attack other crabs.
Simply get 10 crabs (they must get that green aura) and the achievement should unlock.