You should take a look at the iFixIt teardown. They crystal is glued tight to the case, nothing should slide underneath it. To the extent a spec of something did slide under the edge of a part of the crystal that didn't get glued complete to the case, it's no going any further. Then again, this could be the sign of a bigger problem so best to take it to the Apple Store. But don't be surprised when they look at you funny because you tell them there's a spec of dust under the crystal that they can't see.