red bubble algae, remove it carefully. You don't want to pop it since it will spread even more. Get some emerald crabs, they should be able to take care of that
Definitely red bubble algae - everyone here is correct, in that you would definitely want to be careful in how you remove as so you can prevent its spread. I would definitely think that getting something like an emerald crab would do well for you to have something that will do the work for you.
Here is a link to an article about bubble algae that you may find interesting: