AL physical chem!!

They are two different concepts.

The attractions between nuclear and outer electrons are lowered by the screening effect. Effective nuclear charge is the actual part of positive charge of the nucleus that attracts the outer electrons of an atom.
In calculation,
(Effective nuclear charge) = (Nuclear charge) - (Screening effect)

Charge density is the density of charge (positive for positive ion, negative for negative ion) that distributes around the cation or anion. Usually, the charge densities of positive ion are calculated.
In calculation,
(Charge density) = (Charge located on the ion) / (Volume of the ion)