depends on the species, gha is usually taken out by turbo snails, asterias, trochus, urchins, tangs, hermits, lettuce nudis. if it's not too bad I'd say an electric toothbrush will be your best friend. Even a cheap dollar store brush(non metal) will work fine in brushing it off. the algae will go down once the tank stabilizes, but also check your phosphate levels. If it's something like bryopsis, raising magnesium levels will kill it