First! check back in a week and see if this cup is turning into a queen cell.
If it is, then it would more than likely be a supercedure cell. Which means either... the queen isn't present and they are making a new one, or they are replacing the queen that is in the hive. For some reason that we don't know, the bees may not be happy with the present queen, so they will make a new one.
But you should know that bee's make these cups all the time, like they are teaching the new bees how to do it. They build them then take them down. I'm in the habit of during an inspection, I look inside all of them and see if there is an egg or royal jelly. That tells me that the bees are up to something