The framing in this conversation is important. This is difficult, but making it clear you'd rather not see that is how I'd go.
I'd tell them that I noticed I could clearly see the furnature, wall, or some detail and would rather not see them indecent.
No way around it, it's going to be a bit awkward, but telling them shouldn't seem like a weird thing to do. If you're polite, telling them about the issue is a kind thing to do.
Maybe a note outside their door? Could be anonymous if you don't know them well and they don't have cameras, saves both you and them the awkwardness