Q:

Potential following の that follows a noun

顔の見えない誰かに...

I wonder how this is translated as '(to) someone that cannot see their faces'. 
I thought at first that 見えない誰か, as a modified noun, is connected to 顔 with の to mean 'someone that cannot see of faces', which is weird...
I have never seen this pattern before and my research ends to no avail. 

A:

In relative clauses の is frequently used to mark the subject of the clause in lieu of using が.
So, as a stand-alone sentence, the clause would just be. 

顔が見えない。-> I cannot see their face. 

But then the が becomes の when used as a relative clause.   

顔の見えない誰か　-> Someone whose face I cannot see

There are various restrictions on when this can and cannot be done.   One such circumstance when が cannot be substituted like this is when there is both an object marked by を and a subject in the relative clause.