A Green Man.
But I've never seen a Green Man on an 18th century headstone before. The date of execution could be 1703- when the husband died or 1731- when his wife did. The relative sophistication of the design makes me lean to the later date.
This particular Green Man has flowers- probably roses- coming out of his mouth. This also is unusual. Usually Green Men just spew leaves and tendrils and other green stuff. He's to be found at St. Mary's, Barcombe- a mile or so outside Lewes in East Sussex.
Here's the whole, very busy, composition.