What is this?

[Slim]

Ya looks like a retained eye cap. You could get some saline eye drops,real tears works too. Drop on the eye and do this a few times to loosen it up then take a q tip and gently go across the eye, this should take it off. Remember be careful and take your time, gently do this. I have done this before with great success.

I very strongly urge you not to do this! Really bad things can happen when you start mucking around with their eyes. Jack up the humidity to around 70% and let it come off by its self during the next shed.