Why does Juan fear for Mariana's safety even in

"that hidden Paris neighborhood"?

Incomplete question. However, I infer you are referring to the story "The Censors".

Answer:

Likely because of his background.

Explanation:

More likely his background affected his concern for Mariana. Probably be had experienced government censorship of information where he lived, and so thought it was in such neighborhood that Mariana lived.

However, Mariana was in a hidden Paris neighbourhood likely different from Juan's own.