Assumed that a suitable light for macroalgae is already installed in the refugium / sump we would recommend the following marine macroalgae species:

The most often macroalgae species as it is very easy to grow:

CHAETOMORPHA OR CHAETO 

And if you like to create a planted refugium we do recommend the following Caulerpa species, which will need a bit more attention than Chaetomorpha as they can go sexual. See more about this here in the FAQ section.

CAULERPA BRACHYPUS

CAULERPA RACEMOSA PELTATA

CAULERPA PROLIFERA

CAULERPA TAXIFOLIA

CAULERPA RACEMOSA 

CAULERPA SERRULATA

CAULERPA RACEMOSA GRAPE