Looks like mini ramshorns. They are harmless but since they only grow up to a few mm large, they can easily multiple and sustain themselves on just the biofilm in the tank. Most likely hitchhiked via plants/décor from another tank as EvanderT mentioned. I personally dislike them and found no way to get rid of them in my shrimp tank w/ nerites unless I break down the tank completely. Best to remove them while you can or you may have to resort to meds or a loach in the future.
VICTOR PIMP #53 SGT-500-40-4F-2S-DK