Hi,
Thanks for your question. In order to cast the Wurm, it must be in the part of your library that you're able to see when you search. In the case of the Mindcensor limiting what you can see when you search, the Wurm will only be castable if it's in the top 4 cards of your library - if it's not in those four cards, then you will not be able to cast the Wurm.
If you cannot see the Wurm while you're searching for it, it cannot be cast, even if you know it's somewhere else in your library. For example, if your opponent exiles your Misthollow Griffin face down, you cannot cast it from exile, even if you (the player) know for 100% certainty that the exiled card is the Griffin. If the Wurm is not in the four cards you're allowed to look at while searching, you cannot cast it.
Thanks,
Nathan Long
Magic Rules NetRep
Edited Nathan Long (Oct. 16, 2018 09:58:08 PM)