DistinctCount for relatedtable with additional filter
I would wrap your expression in a CALCULATE function, so something like:

activedays = CALCULATE ( 
    COUNTAX ( RELATEDTABLE ( ActivesInactiveData ) , ActivesInactiveData[Surface Date] ) 
    , ISBLANK ( ActivesInactiveData[Topic] ) = FALSE )


I assumed [Topic] is text, so NULL will converted by DAX to BLANK().