Yes, it would totally survive on its own. The bright ones don't survive because they have no green to photosyntheesize. This ones does and will if detached:)

I've wondered why they graft that cactus since it is unnecessary.

I don't know why either I thought the same exact thing. I guess just for looks. IMHO the grafted cactus would be prettier on its own;)

Usually, they graft because the plant grows slowly. If the flowers and seeds are wanted, it may take a long time to get them from a slow growing plant. But I am sure there are other reasons - maybe ppl like the look? - like the Moon cacti...

hi all - thanks for the responses! :) if that's the case, i think i'll leave it graft unless i start to see signs of trouble. i do like the look of it, just want to see that it's not causing undue stress on either plant since they can both technically survive on their own.

do any of you by chance know what time of cacti is grafted on top? i know the bottom is a Hylocereus spp

The one on top reminds me of a Mammillaria cactus.

My guess would be Notocactus scopa for the top plant.

Out of interest, since on this particular grafted cacti both sections can photosynthesise will it last longer than a coloured graft?

It's always confused me why coloured grafted cacti die. Is it because the growing tip has been cut off the bottom part and the coloured top section means new shoots can't grow? I've never had one so I don't know how they work.

The red and yellow and pink can't photosynthesize, so that's the reason they are grafted. They are mutations lacking chlorophyll. If there was at least some green, they would be able to produce 'food'.

They may grow roots and grow for a little while on their own, but usually don't survive for long. When grafted, they are getting fed by the rootstock which is a very vigorous plant, and quite often overgrows the colored one.