There is no particular difference. Chlorophycae Comes under the taxon Chlorophyta.

Chlorophyta is a taxon of green algae informally called chlorophytes. With the exception of Chlorophyceae and others, which show various degrees of multicellularity, all the Chlorophyta lineages are unicellular. 

The Chlorophyceae are one of the classes of green algae, distinguished mainly on the basis of ultrastructural morphology.