I don't knwo the answer, but it should be easy enough to set up an experiment to test the correlation with different volumes of algae. And maybe as an extra variable, do the test at different levels of dryness. Optical Density is defined as the ratio of the intensity of light falling upon a material and the intensity transmitted (https://www.photonics.com/EDU/optical_density/d5788) so has no 'units' and weight is of course g or kg. So, do your experiment, plot OD vs weight at a given level of moisture content and find the trendline for each moisture level.

If you know the OD of a microalgae culture, and you also know the dry weight of this culture, you can make a calibration curve by making different dilutions on which you will evaluate the OD and dry weight.

In this way, when you want to know the dry weight of a culture, you just need to check the OD and go back to its dry weight using an Excel regression equation.

The measure unit is the one used for the weight (generally g/L)

You will find a similar example in the manual of my software (red intensity of coffee vs concentration):

www.lerenisplezant.be/fitting.htm