Let me explain a little better...any old dead coral, shells, and rock that is underwater will form soft growths, and algae on them. This is a common algae that is found in tropical waters...it is quite hardy, and will dry in place without even fading...it can usually be scraped off, but is difficult to remove with soaps and chemicals...it will even resist bleach and heavy cleaners. A green abrasive pad will remove it but it is stubborn. It will stick to most surfaces, even glass, and can build up quite thick and become more even in its coverage causing large areas of red discoloration...I'll try and take a picture of some of it on my artifacts that have been out of the water for years and still have it adhering to them.