I am not a PS wizard and this is a "difficult" masking job, due to all of the tentacles and the traces of backscatter on the lower parts of the image.
I think my first step would be to coarsely mask out the shrimp, bell, and reproductive tentacles (the curly ones), and apply a pretty strong "dust and scratches" filter to the rest of the image. This should be able to clear up a lot of the backscatter. Careful about losing detail in the trailing tentacles.
I would then try to use "select by color range" to get as much of the background as possible. I fear that this won't be quite effective enough -- it might just take some dedicated and tedious selection with the wand tool to get this right.
In any case this won't be "easy", unfortunately.
Dan