no he looks pretty stressed out. T. derasa mantels should extend slightly beyond the shell rim. his looks rather taught.
the inlet siphon doesn't appear to be gaping, but he doesn't look like a happy camper.
it's hard to tell from the pics, but is he on the rocks or the sand? derasa don't like the rocks and prefer to be in the sand bed.
any other indicators not readily obvious in the picture? how is his shell growth edge? how is his reaction to stimulus (shading)? any parasite concerns? visible damage to mantel, shell, or byssus?