Some algae eaters will do that if they're not well fed.  Keeping snails with goldfish is also iffy ,because some goldfish will pick on the snails and eat the antennae.  Keeping up with water regular water changes will help a lot in reducing algae in the tank , because algae require nutrients to grow.  Also make sure you don't have the lights on for too long or else there will be a lot of algae.

You can use an aquarium scraper to get the algae off the walls of the aquarium. This often the best way to keep algae off the glass. There are magnetic scrapers that you can stick on the aquarium that do a good job of removing algae also.

I keep a Sailfin Pleco with my goldies, and he's not bothering them at all
So get one, but make sure it's either vegetarian or well fed

Nossie: How do you know if a pleco is vegetarian?

thanks!

I have Algea eaters in my godl fish tank. They keep the tank clean and algea free. we still have to do regular maintaince but they are great to have with goldfish. we have two of them in our tank. I hope that helps.