strange shapes in unity player

When I play unity games I often get a strange cube which flickers, often obscuring the game action. Can anyone explain this or tell me how to avoid this?

P.S. I have tried uninstalling & reinstalling the unity webplayer & am on a mac

hi
i might have an answer but canyou answer this backÔÇŽI deleted my unity hiearchy and project folders i am kind of new so how would i fix it then ill tell you what you do please respond