The ramshorn snail is an algae eater that will tend to stick to algae unless they become starved. As wiki notes, though, they can end up being a pest in the tank rather than an aid to it if you have more than one.
The nerite snails are also algae eaters, but with the added bonus that they won't breed in fresh water.
The trapdoor snails are algae eaters, they're bigger, and they can give warning of water issues before they become acute. These use to be hugely popular in fish tanks back in the day, you probably saw them in action plenty of times as a kid. They also live breed rather than lay eggs. These may be hard to get, but I would think these are ideal.
In most cases, all of these snails will prefer to algae over other plants in your tank or mini-pond. However, if they run low or out of algae, then plants are probably fair game to them. There are fish species that are far better algae eaters, but as far as I know, none are goldfish compatible.
As a note, there's various beliefs that goldfish can/will eat smaller snails. Dead snails maybe, there's not really any good evidence that they can, or will, eat live snails.