A snail. Or some shrimps.
Nerite snail I suggest, doesn't grow too big and isn't able to reproduce in freshwater. Or some shrimps, depends though, shrimps usually end up as snacks, so test with Ghost (Feeder) shrimps first to see if your Betta is a nice girl. You can't have any fish algae eaters in a 5g, it wouldn't be able to handle the bioload, and the footprint isn't big enough for them.
Remember, algae eaters only eat certain types of algae, so all of your algae won't magically disappear.