You would have to describe the structure of the lichen. But clicking around through the following websites might give you some idea.

Twenty Common Lichens

There are options you can click on depending on how your lichen looks like and the website narrows it down from there.Looking through the pictures briefly I thought that maybe Trentepohlia matched your description the best. Interestingly, it is a green algae (not a lichen species by itself), but it is known to form symbiotic relationships with fungi.

Trentopohlia