The dark marks are under the eye stalks, and they normally have the 'band' around the back of their head too: (easier to see on the lighter hermies as he appears to be as opposed to the darker colored rugs.) In the past year or so, they are not found very often but have been bought by a few crabbers in Canada.
If you look closely on his top torso, you should also see 'dots' that almost form a circle.
How many stitch (some call these 'dash') markings does he have on his large pincher?
As for needs... same as the E's and other species. They too like E's and straws seem to also favor the ocean/sea water mixes so important to offer the actual ocean/sea mixes sold specifically for salt water fish tanks and mixed per the instructions.
If I am not mistaken I think I read in another post where your ocean/sea water lasts a couple of weeks? If so you may wish to cut back on how much you mix at a time. Ocean/sea water mixes have been known to turn/sour after awhile. Make sure to keep a close nose on it. It should have no odor to it, just like the day it is mixed.