plastic interlock screw ? this screw with no head
Those hammer in, so will be hard to remove in a way that makes them re-usable. You could try getting a putty knife or slot screwdriver under the head and prying, but you'll probably make a mess of it.

I've found that the nice people at the Ikea parts counter are pretty good about providing replacements if you show up with the old part (and, as you have, the part number).