My O. Mercatoris was sold to me as "Octopus" I think they are commonly mistaken for Joubini's or just labeled "dwarf species".
From what I've researched, their mantle only gets around 2 centimeters. Einy's is about 1 centimeter, and his legs are probably 5 times his mantle length.
Minimum tank requirements are up for debate, but I have recently moved mine into a 10 gallon from a 75 gallon system. He remains sealed inside a large critter keeper with mesh fixed to the inside of the lid covering the vents. I am running an Aquaclear 50, rated for 50 gallon tank. Its return creats a lot of aeration, there is room for modifying the filter. I keep lots of macroalgae in it and a carbon filter.
They eat Hermit Crabs and Fiddler Crabs, probably ghost shrimp if you had to. I have not gotten Einy to frozen food. He is timid and retreats when he sees my hand. He got curious about a piece of krill I had on a skewer but rejected it. I prefer female Fiddler crabs if you have a choice, as they don't have the monstrous pinchers which are apparently pretty intimidating.
Just do regular water changes, weekly, and run carbon at all times in case he inks. Einy has only inked twice, and he didn't put out a very significant volume. Remove crab carcasses so as to not let them foul up the water. Keeping a tank this small requires lots of attention, so I would recommend keeping it in a 20 or 30 gallon if you're not experienced with small tanks. I will probably add a small skimmer to the 10 gallon soon as it's just best practice.
My observations are that my particular octopus mercatoris is very patient and strategic. He will wait for his food to cross in front of his den. If he becomes imaptient or is hungry he will stalk them and surprise them. Witnessing the octopus in action takes patience as well, because he doesn't get in a hurry to become active at night. Sometimes he is out as soon as the lights are off, but most of the time he sits in his den, with his head popping out. He has not become very predictable. I use a red light to view him after it gets dark, otherwise lights will only make him uncomfortable.
He is still very interesting to watch when he does decide to explore and hunt. He is very stealth and searches out the best place to attack from and waits for just the right moment. He changes from white to red, sometimes displaying skin textures, which I understand is only a trait in young Mercatoris.
As long as you use lots of filtration and I reccomend a protien skimmer as I have suddenly had some strange water quality issues without one.
Most people say they shouldn't be placed together, but Danthemarineman says that he regularly finds them together when he dives and he keeps them together in a tank. I would just watch to make sure they get along and be prepared to pull one out if needed.