The flow rate meter is a white cyclindrical object , and sits between the inlet valve and side matrix. It basically tells the control board how much water is flowing in so it knows when to shut off the valve. If it fails , the valve is left open and the water fills to its top level…..then more and this operates the overfill ‘mushroom’.
If the sticky substance you’re talking about is an orangey sticky gunge that appears to be sitting on the polystyrene float on the base plate , then i can’t explain it but i have seen it before. Best to clean it off to allow proper and correct operation of the flood protection float.