Growth is a Labyrinthula. It's really hard to explain what kind of life form it is (not an animal, plant, fungus, etc) so feel free to try to read the wikipedia page on it, but I will warn you it's a lot of information. Shouldn't be harmful in your tank but some species act as pathogens and have been singled out as the cause of starfish wasting disease here on the eastern pacific coast (your's isn't the same species)