The element doesn't undergo phase transitions, it only changes its temperature from T0 to the equilibrium temperature Te (in this case Te is 0°C for the reason explained later).
Therefore the heat transferred is:
Qelement = melement Celement DTelement = melement Celement (T0element - Te)
(I used m for the mass of element, C for its specific heat)
The water/ice mixture is (and stays) biphasic, therefore at standard pressure its temperature is constant and it is the melting point of ice (0°C).
Therefore the heat transferred is the latent heat of fusion of ice at 0°C:
Qwater = Lwater nwater
(I used L for the heat of fusion of ice, n for the moles of ice melted)
If the system is adiabatic, all the heat exchange occurs between the element and the water/ice mixture, therefore you can equate the two Q's:
melement Celement (T0element - Te) = Lwater nwater
In this equation there is only one unknown, Celement, which is what you want to find.
I hope I didn't make any macroscopic mistake with the maths, but this should be the procedure.