I have a small grey screen on the top left of my computer screen that says "Blocked Plug-in". This happened whilst my children were downloading Adobe Flash whilst in Safari. How do I get rid of it?

I have a small grey, opaque square about 8cm x 5cm on the top right of my computer screen following installation of Adobe Flash through Safari. I don't know how it's happened as my children brought it to my attention after the download. How do I get rid of it?