I don't recognize it. What happens if you quit InDesign and then relaunch it. Does it come back?
Does the box move around when you move the mouse?
If you cannot delete it, it might be caused by a damaged file. Easiest way to get rid of it, is to insert a new spread after that on, select all wanted items and move them to the new spread and delete this spread. Save this file immediately. Export as IDML, open that IDML and save it as new INDD and work from there.
I suggest to do soon as any damage can cause further damage, which can ruin the whole file to make it unusable.