The Yellow Leather Coral, Sarcophyton elegans., has a leathery touch and lack a hard exoskeleton. Like most Leather Corals they may go through periods when their polyps do not extend, and may shrink and shed some of their outer skin. This is a natural process that they go through from time to time. Your Leather Coral may remain closed for several days at a time, but once they open up they will be healthier and appear more vibrant.
As they resemble a Mushroom at a smaller size, they will begin to change into a more folding appearance as they grow larger and wider, therefor it is important to give the Yellow Leather Coral space as to not shade other Corals from their necessary lighting.