Very interesting! It is dark green like phyto!
This caught my attention since I've done a
Blackout, but were talking (90days) 3 months, during a needed Fallow Period (after Velvet) wiped out my ALL fish.
Since no fish...I Moved corals into storage bins, and took advantage of Blackout Opportunity to End years of Alage/Cyano Problems.
Total Blackout. Tank wrapped in black thick Garbage Bags. Threw in a Pencil Urchin, to scape the rocks clean. Sump and Skimmer still running.
3 month later, the rocks were like new, spotless/white (coralline eaten by urchin), but still alive. (I dosed Vinegar during the blackout period)
Best thing I ever did. Tank never had a *(Algae/Cyano) problem ever since, and now it's at least 5+ years later.
The Live Rock is now so Live that I no longer have to do much for this tank other than 2 Part Dosing, and Feeding fish.
I never have cleaned or touch the sump in years. Only Skimmer. Glass I clean about every 6 weeks for extra clarity, but not really bad.
However the Green Skimmage and the Phyto Comment is also co-incidental. I was purchasing Copepods last week, and looking at a Vendors Products.
When reading about their
Phytoplantkon , I notice a particular comment in the product description.
I know that Reef Nutrition have Phyto, and Phyto Live. Always thought the LIVE version was really LIVE. Apparently NOT. It's just more concentrated. (AND STERILE)
This is the supplier that I purchased their copepod bottle.....And This is why phyto in a bottle is STERILE, and
what could be happening in your tank/skimmer.
https://copepods.ca/collections/packs/products/phytoplankton-blend-8-oz-medium-concentration
(
read the reason why they state : This blend is sterile and will not reproduce. )
IT SOUNDS like you somehow introduced, or had NON-STERILE Phyto in your tank, and right conditions + LIGHTING triggered a Phyto Bloom.
If true, you could bottle your green skimmage and sell it to folks who need Phyto for their Copepods. (would need to be sterilized first)

Kidding but possible.