Sure? I think so

I sent to algae pictures to the tree researchers and they said, yes, that's what it is.

Then I went along to the Wayfinder Oak and had a look at his signature.

Well, I thought I could see that the algae was indeed making the tree red, but there is still the fact of peculiar striations on the bark that seem to exist in coincidence with the algae. Or that the striations make the bark especially habitable for the algae for some reason.

I inspected the Goddess Oak too, and it seemed to me that the striations were places that algae like.

That means, there remains a question: what causes the striations?

On the Wayfinder - and I have seen this in other cases - they are by a sort of "fold inwards". Often they are in a kind of "under arm" position, below branches. On other occasions, they seem just randomly on the bark.

The first two suggest that some inner force of growth might cause the striations. As for the other? No idea.