Yes, that screw clamps the strap.
If you can't move it, I recommend "electronics cleaner". Triclorotrifluoroethelene. It's also sometimes referred to as tuner cleaner, back when TVs had mechanical switches to change channels.
Be careful when you turn the screw, you can strip it very easily. Gentle is the key, here.
If none of that works, you can force it out of the clasp but that is definitely your last resort!
Tom