the 2 large rocks on the right look like they have no coral on them, just simply immerse them in boiling water and scrape off all the algae. literally remove it by force. and reintroduce to the aquarium.
if you want to go another route you can just pick them off without bursting them. I used to have them in my old reef tank like marbles, they do look attractive in small amounts.
you just get a firm grip and twist them off.
ps. very nice looking natural nano. the clowns look happy and there is nothing wrong with your tank. I would be proud to call it mine. but there are a lot of people on the internet who take marine aquaria to a whole new level of perfection. which is perfectly fine, because they know what they like and they try to recreate that. but if you are happy with your tank you have nothing to worry about.