This can happen sometimes if your Slidetile is left on the magnet for an extended amount of time.
But not to worry!
The mark you see is just residue from the UV coating on the magnet. Let your Slidetile sit for a day or 2 in the open air and the mark will disappear.
If for some reason the mark doesn't disappear, please send us a message with a photo of the damaged Slidetile and we'll send you a new one free of charge! :)