Goldfinches feeding on moss or lichen?

There has regularly been twelve or more goldfinches at my feeders. This morning the number increased to twenty five and many of them spent much of their time on the roof of my neighbour's garage. The tiles on this roof are largely covered in what is either moss or lichen and these birds spent much of their time apparently eating the lichen. This is not the first time that I have witnessed this. If this is moss, there may be seeding heads that the birds are eating. Owing to the position of the roof it is difficult for me to get close enough to definitely ascertain what the plant is.