this happens when geckos hang upside-down like your gecko is doing. this is called FTS (floppy tail syndrome). it does not affect the overall health of your gecko. there is a somewhat debate on what causes this, some people believe its because of deformed pelvic bones. while others just think it's weak tail muscles. this is acually common in cresteds. just be sure to be supplementing calcium. if you use CGD, you do not need to supplement with calcium.
It's not uncommon at all for them to hang upside down. I agree with Geckoguy, it looks like he might have FTS. I don't have any experience with it myself, but I've read a good deal on it and I don't personally believe that it is terribly harmful to the gecko, but it is something that should not be avioded, as it is by no means healthy. Add branches and stacked egg cartons to the enclosure; give him plenty of things to climb so that he uses the glass less often.
It's not pellet like... It's a powder made by repashy specifically for crested geckos. It is really good, I feed all my cresteds only CGD, nothing else.
CGD is also made by Komodo (although it tastes nasty) and T-rex as well as Repashy. Repashy seems to be the preferred one, it comes in all sorts of nice flavours.
