That looks like Aiptasia, not algae. GHA usually vanishes completely when it’s grazed down, but when something “underneath” remains with a translucent, fleshy look and little tentacles, that’s the giveaway. It’s common for hair algae to grow around them and hide them until the algae is removed. I’d treat it sooner rather than later, since they spread fast. Spot treatment with something like kalk paste, Aiptasia-X, or removing the plug and treating it outside the tank works best at this stage.
Looking at the plug at the center of the picture: the clove polyps are definitely supposed to be there, but the gentleman in front of them I thought was GHA until something else mowed away the actual GHA and revealed him underneath. A Google search has proven fruitless. Any idea what species he might be?
