What is the laws of cause and effect in Mirai Nikki?
I think it's just refer to the general Causality concept.

From Causality's Wiki page


  Causality (also referred to as causation) is the relation between an event (the cause) and a second event (the effect), where the second event is understood as a consequence of the first.


There's also Causation Wiki page

From God and the Laws of Science: The Law of Causality


  
  “the principle that nothing can happen without being caused” (“Causality,” 2009). 
  “the principle that everything has a cause” (“Causality,” 2008).
  


and The Principle of Causality


  Nowhere in the world can there be any phenomena that do not give rise to certain consequences and have not been caused by other phenomena. 


And according to the link you gave, even the room where Deus located is called Cathedral of Causality

Hope those above answer the first question.

For the second question, I guess you refer to the content of the law? As far as I know and understand, it's simply the same as its definition:


  “...nothing can happen without being caused” (“Causality,” 2009). 


Hope this helps, and sorry if I'm out of topic or get your question wrongly.