Best guess based on your photo is that you need a light source to detect and a shield around the detector to block out ambient light.
You could either do beam-break detection where your light sensor is on one side of the slot and your light source is on the other, or you could do a reflectance sensor where the light source and detector are on the same side. A beam-break sensor detects when the coin blocks the light, and a reflectance sensor detects when the light bounces off the coin.
The shield just limits the light that the detector can see. It's just like looking through a paper towel tube, the longer the tube is, the fewer light sources you can see. It helps if the inside of the tube is not reflective. A rolled up bit of paper works pretty well for this.
I'm not sure if a hall sensor would work for this. I know it will detect iron, but I'm not so sure about the non-ferrous metals in coins. I think you'll be better off with the optical sensors.