My guess would the be small, nocturnal (eyes) we see from the Philippines that is likely in the Abdopus family (arm length and coloration). The eel is not safe with the octo. You need a secure top on the tank (the lower water level will not do much to keep the animal in the tank). You may want to put a red light on the tank at night to try to observe it.
I agree with D on the id. Not sure about the un/safety to the ribbon eel. I've seen them share holes temporarily. They don't seem to have any natural predator-prey relationship even though they co-occur. Who knows about sharing a tank though... That's tough to say how that will go.