I saw on tv that the next episode of survivor has a muddy challenge. I have looked and looked for the episode preview but can't find the same clip that I saw on live tv.

This scene aired earlier in this season, maybe the first or second episode...probably why you can't find it in the previews. Like a lot of the mud coverage on this show, the consistency and quantity is amazing. Unfortunately for us, there's almost never any "great" messy content on this show, because they cut the competitions together to make exciting action scenes, instead of WAM fantasies.