lichen doesn't matter and the slates won't be pushed off/aside by the moss. However if there are great big cushions, then in winter the moss holds a lot of water which will freeze and expand which could cause some damage long term.
Beware of pressure washing unless there is a waterproof membrane underneath, best way to remove it is a proper roofing ladder and a stiff brush, unless it is like something out of the Lord of the rings...:-))) some people also use a metal strip, think zinc or copper to repel the moss.
The moss can only grow where there is dirt, unlike the lichen, so keeping the roof cleanish will help to stop it returning.