Temporary veneers - there is a layer of cement/glue on the gums, should I remove this? (photo)

This is excess temporary material and not cement that you are seeing.  I would get to your dentist and have them remove so that it does not irritate your gums making it more difficult to bond in your permanent veneers.  It will be fairly easy to get off.  

Hello there,It appears that you have excess temporary material on your gums.  I would not recommend removing the excess material yourself.  See your dentist as soon as possible to have it removed.  Waiting until you next appointment could result in your gums becoming inflamed.  Good luck and I hope this helps. Follow me if you have more questions.Sarah Thompson, DMDSt. Louis, MO area (O'Fallon, IL)Real Self 100

This should be removed. Call your dentist and let them know and they'll have you come by for just a quick appointment to remove this excess material. This will allow your gums to be as healthy as possible in the temporization phase.Douglas Jopling,  DDS Dallas,Texas area

You need to return to your dentist and have them contour the temporaries. You should not pick this off yourself. leaving it on for several weeks will cause your gum to become inflamed. I suggest getting in to your dentist asap. 

I don't suggest YOU doing it in case it chips off the teeth as well.  Ideally it is polished off.This is likely not cement but the temporary material itself.  When first placed was likely transparent and overlooked.  Over time it became more opaque and obvious.  If left alone, bacteria can harbor and irritate the gums, so it is best to remove it.