Don't be surprised if your seahorse winds up being the first meal. Makes for an easy target and a lot of octopuses love to munch on anything that moves. Not always the case, but I would be cautious not knowing what kind you have.
We need information. Like, how long have you had it? What are your water parameters? The restlessness combined with lack of appetite is worrisome. If your water parameters are out of wack then the restlessness could be a sign of discomfort/distress, this would also explain the lack of appetite. Could just be a senescent octopus doing what they do. Old male octopuses can act nutty and refuse food before they die.
Easiest way to tell is enlarged suckers IMO. Suckers on females arms are pretty uniform, don't have much contrast in size whereas males tend to have obviously over-sized suckers on the bulkier part of their arms. Males 3rd right arm is modified, but it's not too easy to tell when they're moving around constantly. Some males seem to carry it curled up most of the time.
From the second picture it looks as if the octopus is maybe missing the end of an arm? (directly right of the eye)