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Its a very healthy green hairy mushroom, some of them get up to over 6" in size. Move it into the light and it should also take food when you feed the aquarium.
Its going to do better with planktonic food or thawed brine shrimp/brine shrimp juice. Keep in mind that it doesn't have to eat, as it is feeding on the photosynthetic algae that live within it as its primary food source.