Yeah, we have known that WoB for a while.
I come bearing some more WoBs:
Also, there is another WoB about Taldain Investiture:
The lichen on the sand uses Invesiture in it's lifecycle - when it gets water it releases it and reproduces. So I doubt there's much keying in to the lichen, the restriction - if there is any - is on being able to form a temporary Connection with the lichen. Sand Masters can do that to feed the lichen water from their bodies and when it releases Investiture, they can use it to manipulate the sand.