Algae but only on rock?

[TheNoseyProject]

I'm still establishing my tank so it currently has 4 mollies, loads of tiny snails (as one got in off a plant and they breed like crazy) and live plants. I cycled the water for 6 weeks before introducing fish 3 weeks ago.

The water is clear and the fish (and bloody snails) well, but my big rock in the centre is turning more and more green. Any suggestions how to clean it? Even scrubbing doesn't seem to shift it. I have some aquarium cleaner liquid but it hasn't done much so far.

Here's a pic

I'd be happy to have some algae in my tank as a natural food source for the fishes- mine doesn't have any atall?
But if your not, have you tried taking it out and putting it in a bucket of boiling water? Not entirely sure if it will work but it's worth a try! :)

Do you have a scrub brush? That’s what I use to scrub rocks