I think it is normal when I had a bunch of zoanthids they were there and when my colony eventually died it disappeared not because of the grey thing I had the colony for a while and it was infested with tiny nudie ranches that only ate ate zoanthids. For my Current jasmine polyps they spread like wildfire and just before they spread to some where new the grey looking thing goes over there then they pop up. So I think that there way of reproducing or etc/ spreading. Ian not sure this is the same thing but just trying 2 help