Those cacti are grafted since they don't do very well on their own - lack of chlorophyll in these plants make them hard to live, if at all. So rootstock supplies what graft can't get on its own.

Even if they live on it's own, it is usually for just a short time. Same happens to many pups of other succulents that happen to grow in similar way (lack of chlorophyll makes plant to grow white/very pale yellow leaves) - if separated from mother plant, they often die.

Is there a fertilizer or supplement I can put in the soil?

Why? - because you think plant needs it or because you think fertilizer will assure that plant will grow?
If you want to fertilize plants to give them more 'food', you could use a well balanced fertilizer; using about 1/2 of recommended dose is better than going heavy all of sudden.
FoliagePro 9-3-6 fertilizer seems to be one recommended.

A slow release fertilizer could be added to potting mix too.

Many ppl don't fertilize succulents, other do.

I looked for some pics of coral cacti. Technically not cacti but looks like the scion has some green in it so it might be able to survive without a different rootstock. However, if it can survive well without a different rootstock then I doubt the nurseries producing it would bother grafting it. Rather than cutting off the whole crested scion you might watch for a small outgrowth of it, cut off the outgrowth, and try rooting it. That way you'd still have the original plant if the cutting fails to grow on its own.

Most grafted moon cacti have no chlorophyll and won't survive on their own. However, I've seen some scions that have a mix of colors, including the normal green, and some that are very dark purple. The dark purple ones might have chlorophyll. The ones that have a mix of colors including regular green might be able to photosynthesize enough to support the rest of the plant. The brighter solid color scions have no chlorophyll and can't photosynthesize. As far as I know no amount of fertilizer or supplement will make up for the lack of photosynthesis. Without a photosynthetic rootstock the ungrafted nonphotosynthetic cactus will die. Often the scions will produce pups. You can try cutting them off and rooting them so you'll always have the original grafted plant. I expect you'll find the cuttings don't grow and eventually die.

I know not to go heavy on fertilizer. It's a treatment to help grow not a need. I've been growing veggies for years so green thumbs are loke a back hand to me but I'm now starting a spiked green thumb. I'm going to do some research on my own and see what I can pull up. Thanks a bunch for the info you've helped alot and it's very appreciated. Wondering how moon cacti and coral exist in the wild

Crested Euphorbia and cacti naturally happen. The abnormal growth pattern isn't
lethal but probably doesn't help the plant grow well in its native environment. Moon cacti without chlorophyll don't exist in the wild. Or, if one germinates from seed in the wild then it doesn't live very long. Lacking the ability to photosynthesize is lethal for most plants. The exceptions are holoparasitic and holosaprophytic plants. Naturally, cacti are neither. Grafting a moon cactus on to a photosynthetic root stock is like forcing it, by human intervention, to survive as a parasite.

So do you think if I take a cutting from the coral cactus I'd be able to let it callouse over and pot it?

That might work. I haven't tried it. However, it probably won't grow as well as a grafted plant.

Yes, you can grow Euphorbia lactea crest (coral cactus) without a graft. You can also take cuttings and root those. This is a slow growing plant, but as Polypompholyx says it may grow better with the graft. Search this forum and you will find many threads on this plant.
Search for Euphorbia lactea crest

You can also search Google or Bing images. Most will probably be grafted, but there are some that are not grafted.

My plants are still grafted, but I have one with a split in it that tells me I may be at least taking a cutting in the future. Split coral cactus thread

Good luck and have fun - whatever you decide to do!