We encountered a forest which was difficult terrain. Using the Blink spell a character appeared in the ethereal plane and moved some distance.
My warlock with poor stealth spotted a group of centaurs deep in the forest. Rather than getting too close, he moved behind a tree for total cover.
Curious about the centaurs, he wanted to get closer without being seen. He cast blink; on his next turn he readied movement. At the end of his turn the warlock appeared in the ethereal plane and used his readied movement to approach the centaurs to see what they were up to, without being seen.
We didn’t know if the warlock’s movement should be affected by the thick forest underbrush when in the ethereal plane.
Is a character's movement in the ethereal plane hampered by naturally difficult terrain from a forest?