[Aquattro]

Josh, there are products that will help to a certain degree in the removal of cyanobacteria, although they are only band-aids. Finding and eliminating the source is the preferred method of treatment. Check bulb age, adequate circulation and nutrient removal. Water changes or a quality skimmer will do that for you. Siphon off as much cyano as you can with each water change; it should come off in sheets.
Are you using RO water? You should be. No point putting in what you're trying to take out, eh?

As for the seastar, yes, it will eat the critters in your sandbed. You can go to reefcentral and discuss this with Ron Shimek to obtain a valid answer.
Cucumbers are good at sifting sand, but they're also good at killing everything in the tank if they die or become stressed. An assortment of cerith and nassarius snails will sift things well for you.
Most (all?) sand-sifting fish are sifting to find bugs to eat. Bad thing. Conches do a good job as well.
Your first priority should be removing and preventing nutrients from entering the tank. Make sure your bulbs are new and evaluate the current in the tank. Maybe add another powerhead.
Remember, only bad things happen quickly. Have patience and a plan, and you'll get thru this fine.
Good luck!