Mexican red leg hermits will help you out. Not only will they eat cyano (seriously) but they will eat uneaten food which will sit on your sandbed and fuel the cyano.
You have pretty good skimming for a 50g, so either the injectors need cleaned if you're not pulling much skimmate OR (most likely) you have a couple other things wrong, like yote pointed to.
1. Evaluate your water source. If you're using tap water, then you're fighting a never ending uphill battle. To not get too in depth your water company put in chloramine which eventually break down, giving you a small amount of ammonia which will turn into nitrates. Secondly, in order to protect their pipes, and save them money, they include ortho phophates and hydrolisable (sp?) phosphates into the water which will, in your tank, break down into simple phosphates. If you're using RO, consider RO/DI, either way I would consider taking a sample of my water to a decent LFS and have them test it with a TDS meter.
2. Like yote said, FLOW! Flow will keep food suspended in the water column. This will mean it won't sit and break down on the sandbed, and that it can be mechanically filted out.
3. Light. I'm assuming your MH bulb isn't that old, since you claim to be a noob. How long per day are you keeping your lights on? If this is a new tank, I would advise somewhere in the 6 hour range, and slowly work your way up to 8-10 hours at the max. There are many good articles on RC about people stunting coral growth with too drastic of photoperiods, so don't be worried if this sounds like it's not that long.
4. Watch your feeding. Make sure you're feeding a high quality food. Cheap stuff is packed with phosphates. Cheap brine shrimp is just loaded with phosphates, so always strain your frozen stuff before tossing it in your tank. (brine is horrible anyways.)
