Ive heard of this happening with too much moisture, and a weak end of the egg will rupture. It happened to me once with a panther egg, and it was about 4 months in, and ruptured on one end, but that egg was poorly calcified, and the shell just couldn't handle the pressure of the growing neonate. I expected it to go bad after it was laid, but it held in there. There really isn't much you can do but leave it in there and see what happens. If it starts to get moldy, you know its probably bad. If they other eggs look fine I wouldn't worry about it honestly. This stuff happens sometimes. Also I will note that sometimes when eggs sweat, the liquid can be a yellowish hue, but what your describing doesn't quite sound the same. Take a picture, and we will have a better idea of what might be happening to this egg.
There is no fuzzy stuff. The dirt they laid in was very dark and stained the chams (they had to shed it off) and stuck to the eggs making them dark. I have checked them many times. I now use sand and the eggs stay white.