Lichen is an organism of a fungus and an alga combined. It's unsightly, obscures the writing on the headstones, and worse, if left unchecked, can cause pitting of the stone from secreted acids. They can also take "root" and those roots will swell up with moisture that can cause detrimental cracks.
There is a huge difference between a natural patina that comes with age and just plain old dirt, grime, and staining.
Decomposing leaves and plant trimmings can cause significant discoloration and decaying residue on the stone if left unattended for years on end.
It's gross and unsightly to be sure, but bird droppings pose another danger. Marble stone, in particular, is soft and porous and bird droppings can cause yellow staining.
Tree sap, in all its irritating stickiness, can cause discoloration and staining of the stone.
Let's Be Honest...
You've probably never considered a professional cleaner for a loved one's headstone before but you really should. Headstones are an expensive investment but proper care can extend the lifespan of that headstone or monument. It's a loving gesture that provides the tender care and attention they deserve.
Headstone cleaning can prepare gravestones ahead of holidays, for death anniversaries, remembrance services, out of state family visitations, or before a secondary burial occurs.