Just found 3 of these sea slug/ cucumber/nudi looking things.
Hasn't got the frilly Gill things that nudis have on their back so probably rules that out.
It's on a gsp that just bought and camouflage quite good same colour as top of gsp frag like browny/ white,2 are very small like 8-12 mm long and other one looks like got spines on it and little longer