The description you provided points to a euglena, a single-celled organism belonging to the phylum Euglenophyta. Here's why:

* Nucleated: Euglena have a nucleus, which contains their genetic material.

* Green: They contain chloroplasts, which are organelles responsible for photosynthesis, giving them their green color.

* Moves by means of flagella: Euglena have one or two flagella, which are whip-like structures that propel them through water.

Therefore, a nucleated green cell that moves by means of flagella is most likely a euglena.