From the Picture Stockton Archive.....
https://picturestocktonarchive.com/2007/01/17/skull-and-cross-bones/
If the churchyard you visited is St Mary's on the Green at Norton, here's your answer....
"At last I have the definitive answer to this mystery headstone. The explanation was published in the May 2012 edition of the Norton Parish magazine. The occupier of this grave is Jeremiah Moore who died on 20th July 1753 aged 57. He was a slave aboard a Turkish galley before being rescued by a British man of war. He was then press ganged into the Royal Navy where he served for a number of years. On his release he came to Norton where he entered fully into village life. On his death he left an inheritance to six local gentlemen. The skull and cross bones on his gravestone is not the sign of a pirate, but is an 18th century sign indicating a belief in resurrection and life after death. It is actually called a ‘Polycarpus’."