Best to pull out the rock out of the tank and scrape it off with a razor. Then rinse with ro water.
Emeralds will eat them if there small bubbles.
Do not pop on in the tank. then the spores go everywhere.
Remember you are feeding the algae for it to grow. It consumes nitrates, phosphates and light for photosynthesis.
Run a macro algae, phosban, do water changes and test nitrates and phosphates. Cut back on feeding and light. Look at the amt of phosphates that are in the food you put in to the tank.