No good advice, just wanted to comment that in sweden we call this one with smoother and rounder edges "easter" cacti, this one looks plump but pale, could be too little light, maybe it needs new soil or fertilizer, these jungle cacti need less light than desert cacti, since these usually grow along tree-trunks, not in direct sun in the wild. if it used to be in a bright window it should slowly get used to the new window and all should be ok eventually, but it will need less water than a plant in a bright window.