Are they almost clear? I have had some in my tank for years that are white, but very thin, and the color of the coral under them shows thru. I had not seen them for several months, but I say a few just tonight on my ricordia.
Everything I have read says not to worry about them, they are not harmful and their populations tend to rise and fall.
I used to worry, and dip the coral everytime I saw one on my hammer or ricordias, or whatever, but they have never caused any damage