I wouldn't let Woody eat the lichen, I would scrub it off the perch just to be safe.

I believe that Rowan is a safe wood for cockatiels yes. Another name for Rowan is Mountain Ash Tree.
http://en.wikipedia.org/wiki/Rowan

And the Mountain Ash Tree is listed as a safe wood for cockatiels on the link below.

http://www.avianweb.com/safewoods.html

 

I know of No Danger from lichen, so I would not worry about it.
I certainly have never cleaned it off, I let the birds do that.
A lot of things like this and the bark of many trees are far better for your Fids than man made supplements many use.
As always I put my flock up to show how well they can be if feed on natural foods and left to live on natural things we find in life.