Small lump on the back of my neck near my spine. several months ago it popped with a soft material. will not go away and is under skin, painless, moves?

Epidermal cyst: Epidermal cysts are benign, but they are a nuisance, because they periodically go through the cycle that you described. Eventually it will swell up and pop again, discharging evil-smelling toothpaste-like stuff, and later on it will do it again... So you should have it removed. Any dermatologist can do it in the office with local anesthetic. Minor surgery. No hurry.

Posterior neck lump: Possibly a sebaceous cyst (aka epithelial inclusion cyst). Was the expressed material cheesy & malodorous? Such cysts require complete removal including capsule to prevent their recurrence. See pcp or a general surgeon for this procedure. They will first evaluate to make sure no vital structures are involved.