I doubt that it is contamination. Looks like something has crystalized.

I strongly recommend Maureen Lynch answer.

Their do not look like any cells that I ever seen.

I agree with Maureen, squared objects are crystals!

definitely cystals

I've seen many of those in my cell cultures when I add the medium to the cell culture too late, or even when the concentration of my growth factors was not balanced. You should reconsider what you are adding to your medium. Hope it helps :)