Hi Wes.
Firstly let me tell that you have made nice pictures.
Now back to your question.
Without knowing systematics of algae on the picture I can only make an educated guess.
Most of eukaryotes will use Chlorophylls (a, b and some others)
Here some examples of spectra from light harvesting protein:
![Image](https://d3i71xaburhd42.cloudfront.net/615393c311e4eab1c45fe9130470049be50eba03/4-Figure1-1.png)
Chlorophylls will absorb light at at around 400 nm and around 690.
In between there is so called green gap, where chlorophylls do not absorb. To some how compensate for it plants an algae use many different carotenoids, you can see them as bumps marked by arrows. But they rarely cover this range fully.
So if you excite around ~490 you get much more signal then when you excite at ~547. And in example above Chromera velia will give stronger signal then the other two.
Red algae and cyanobacteria contain phycoerythrin, and its spectrum fill look like :
![Image](https://upload.wikimedia.org/wikipedia/commons/thumb/0/09/RPEexemComparison.png/1024px-RPEexemComparison.png)
They will absorb much more light at 547 then some green guy.
There is for example spectrum of synechocystis:
Another point to take into account is the power of illumination.
It will depend on your light source and excitation filter you use.
I can not comment on your filters, but tungsten halogen lamp will have considerably more power at 549nm than at 490nm.
I hope it shred some light on the question.