That "marble" under your skin isn't tension. It's a knot — and it's physical.
You know the spot. Base of the neck, top of the shoulder, that hard little lump you can press with a finger and go "there — that's it."
That's not vague "tension," and it's not just bad posture. It's a trigger point: a band of muscle fibers locked in contraction, gripping and refusing to let go. A physical thing, in a physical place — which is exactly why a stretch or a good night's sleep never quite resolves it.
It also means the spot can be worked loose. But only by something that actually gets into the muscle and does the work.