A Covering of What?

I am rather baffled by this grave in the graveyard of St Leonard the Less in Salmesbury, Lancashire. It marks the last resting place of the Reverend William Scholes who died September 7, 1913. He arrived at the Vicarage in 1881. It looks as if it had once been buried or, perhaps, coated with Ivy. The residue covers most the the stone but has been scraped away in places. I am left wondering what caused it.