There are two types of moldy eggs, those that candle good and have high chances of going full term and those that no longer have veins (those will develop a very foul smell), those are obviously dead.
Had a similar situation with my solitary het pied egg which sadly has definitely gone bad - I'm actually a little afraid to open the tub to look at it now! It started with a little mold but lost it's veins fairly quickly and within a few days it was pretty obvious that it wasn't going to go full term. My theory is that the egg actually died and went bad as a result, rather than the other way round. This was at about day 35.

From the pic you posted, the mold does look a bit scary, but overall it looks a lot more healthy than my dud egg! Really hope it goes full term for you; I'm sure you're doing all you can, and it is heartbreaking to lose an egg, but remember sometimes things just aren't meant to be - 1 egg in my fire clutch has lost it's veins completely after about day 25 - the egg looks fine to the naked eye, nothing I can do about it though