I think it is a Euphorbia tirucalli commonly known as Firesticks,Sticks on Fire etc. Goes a lovely red colour when stressed,hence the name. Not a cactus but a succulent.
“Every day is ordinary, until it isn't.” - Bernard Cornwell-Death of Kings
I would treat it like any other succulent. I assume you've got it in cactus compost? With drainage holes in the bottom of the pot? Stick your finger into the soil below your rock mulch, down several inches.. and if it's dry, I would give it a soak.. then leave it a bit. Does it get direct sun? I would worry the black stones would heat it up and kill the thing.
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Goes a lovely red colour when stressed,hence the name.
Not a cactus but a succulent.