Beginner Problem II: Square in the middle of the export

Posting a cropped screenshot does not give us any idea what the structure of the composition is. Set your Composition Panel to a magnification ratio of 100% and the Comp Resolution to Full, look for the white square, and then start turning off layers one at a time until you find the culprit. 

If that does not fix your problem, Embed a screenshot of the whole UI instead of Attaching it by just dragging it to the reply field so we can see it without downloading it. Maybe we can help you figure it out. 

If that fails to reveal the problem, try pressing the 'u' key twice to reveal all modified properties of every layer and start turning off effects one by one or resetting values until the problem goes away.