Any bagged live sand should be fine. Merc's aren't diggers so it won't be spending a lot of time messing around on the sand.
Try using a turkey baster or small pipette for the cyclopes-eeze. Merc's have a tendency to sit with their front arms looped up over their head, between their eyes. This is possibly filter-feeding and would be the ideal time to add the cyclopes-eeze.
You should consider ordering fiddler crabs from
www.aquaculturestore.com they are MUCH cheaper than Emerald Crabs and not considered to be any less desirable of a meal for an octo. Just make sure and disable their big claws!