Some slugs can have that "spot" in them. A yellow color is usually indicative, of a slug, as well.
What you would want to see is some veining and something clearer/lighter than the yellow displayed in the pic of the candled egg. Some pinkish color, when candling, is another plus.
Veining, in fertile eggs, can start out with just a wee bit of veining (that is not always easy to see), which increase over time, or they can start out, at the opposite side of the spectrum, with a ton of veining from the start.
If you don't see much veining, at the start, candle after about 2 weeks and veins should be quite evident since they will have increased.
The "egg", in the candling photo, looks like a slug to me.
You can still incubate, if you wish, to lay your mind at ease, in that they are slugs, or, who knows, maybe you get lucky in that I am wrong about all of them being slugs.