As long as the spots don't move around they are almost certainly algae, soft algaes are easy to remove but if it is tough and gritty to the touch you can scrape if off with a credit card edge. It is best to siphon it out once scraped off as it sometimes reattatches to whatever it lands on.
Use a good quality food made for goldfish or koi. don't use food made for tropicals as a staple, it is to rich for them and can lead to bloat. You can supplement the goldfish food with occasional algae wafers, spirolina, nori, blanched spinach, blackworms, and chopped earthworms. They also will eat live brineshrimp.