Hi !
I will check the literature to see if there any (obligate or occasional/facultative) associations between Periclimenes spp. and upside down jellies (Cassiopeia I guess ?), but it's true, it will be easier with a clearer picture
was this species ALWAYS associated with the jelly fish ? or just once ?
do you have a shot of P. rathbunae ? Debelius, 2000, p. 28 published 3 pictures of P. rathbunae, including a shrimp with red-spotted legs, and noted that the colour is determined by the age and by the host (which is possible); anyway, the general pattern of P. rathbunae seems to be slightly different
cheers
Art