To tame this crab you need to locate and click on five large, yellow Kaja'mite Chunks in the vicinity and then quickly tame him before the effect goes away. If you try to tame him without clicking on the kaja'mite you'll be told you're unworthy.
Level scaling: In Shadowlands most NPCs will scale with the Hunter's level, within the constraints of their level range. Hunters can tame regular NPCs up to 2 levels higher than them, but can only tame elite NPCs of the hunter's level or below.