I think I can imagine the shade of ‘crab-green’, a deep dark place where this wily old conger eel escapes its enemy the seal.  Jarrk himself is so strong that he has no enemies, allowing him, in Williamson’s take, to be gentle.

Except where it comes to eels.

Elsewhere a startlingly ugly sea-bird, the shag, is reduced to a ‘crab-nibbled corpse’.

‘For whenever it saw Jarrk the seal, its enemy, it hid far down in the crab-green water, in a hole in the rocks of the deepest pool.’

Source: Henry Williamson, Tarka the Otter: His joyful water-life and death in the two rivers, illus. C.F. Tunnicliffe (Harmondsworth: Puffin Books, 1976 (1927)), p. 93