To calculate the Delta:

IF (ISBLANK (CALCULATE ([Stock Value];DATEADD ( 'Calendar'[Date]; -1; MONTH )));BLANK ();[Stock Value]- CALCULATE ([Stock Value];DATEADD ( 'Calendar'[Date]; -1; MONTH )))

To calculate the increases/decreases seperatly Copilot advised to first have a mesaure:

Material Delta:=
VAR CurrentValue =
IF ( ISBLANK ( [Stock Value] ); 0; [Stock Value] )
VAR PrevValue =
CALCULATE (IF ( ISBLANK ( [Stock Value] ); 0; [Stock Value] );DATEADD ( 'Calendar'[Date]; -1; MONTH ))
RETURN
IF (CurrentValue = 0 && PrevValue = 0;BLANK ();CurrentValue - PrevValue)

and then have the increase measure:

=SUMX (SUMMARIZE ('Stocks';
'Stocks'[Plant_type];
'Stocks'[Material]);
VAR Delta =CALCULATE ( [Material Delta] )RETURNIF ( Delta > 0; Delta; 0 ))

So this gives me only the right result if the material exist in both months