If you get too close to another user in VR, they may appear transparent. This can be changed on your end. You can do this by opening your Watch > Settings > Experience, and scroll down. See this article for more information.
Another reason that someone may appear transparent is that someone has muted or blocked you, or that you have muted or blocked them. You can unmute and unblock someone by going to you watch > People > select the person > unmute / unblock. If someone else has muted or blocked you, they will have to do this on their end.