Wow. I have never seen that. Kinda cool how it just spread like that I wish mine would do that lol. Maybe you could scrape and vaccum the pieces off the rocks to try and remove/control it.
WD is extremely hardy. Only time mine would get that bad in the past was when I let the tank go on auto pilot with no water changes for extended periods of time and nutrients would drop to true zero. I do know if you want to color it up you really need no3 and po4 or else it just stays a greenish yellow.
I would try using someone’s low range no3 test kit and a hanna ulr po4 tester. The low range tester is not nearly as accurate since it measures in ppm the ulr measures in ppb.