I've seen this happen with WA explorers/tourists from homeworlds, although if it's the same thing I would describe it more like the sims are wearing something akin to graduation robes that are not quite transparent but maybe translucent. Can't quite see their naked bodies through them, but it's kind of close to that. I believe the issue is widely attributed to "bad CC," although others here might be able to provide a different explanation. In my case it's a bit distracting when it happens but not impossible to play through.