In five-kingdom classification, unicellular green algae are included in the kingdom
According to NCERT Book (Chapter: Biological Classification, Page No. 18), Chlamydomonas & Chlorella (Examples of Unicellular green algae) are in Kingdom Protista as both are unicellular and eukaryotic.
Please note that Blue Green Algae(Cyanobacteria) is in Kingdom Monera. (prokaryotic)
Multicellular algae are in Kingdom Plantae(Metaphyta).
So, Algae are present in three kingdoms in 5 kingdom classification.