What is this bright green spike on my cactus?
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That is an Opuntia, and the new growth is badly etiolated. It will never fill out to become a proper pad. You can remove it at the base and even use it as propagation material). The base will grow out again, but this time make sure it gets at least 6 hours of afternoon sun daily.
It almost looks like a piece of Austrocylindropuntia subulata grafted to some other Opuntia. The "leaflets/spine sheaths" are pretty unique to A. subulata, not a normal Opuntia thing.
Especially when you consider that the Tephrocactus articulata inerminis (the pine cone looking things) in the same pot look like they get plenty of sun. And the plant in the pot behind doesn't look sun-deprived either.
Question: was this green thing on this plant when you got it? Has it always been in this spot or did you just move it there?