This damaging of art and history in the name of climate awareness has been going on for months.....

[ProfessorGAC]

11. It's Probably Just Carbon Black

VERY fine graphite powder.
A pound of that would turn a lot of water black.
Because the particles are so tiny, the surface to mass ratio is very high. So, they'll float for quite a while before settling out. It'll take even longer if the fountain continues to run.
And, you're right about the toxicity. It's exceedingly low. If ingested, they wouldn't even pump the stomach.
The only real health concern is long term inhalation.
However, filtering it out would not likely be done by the pumping system's intrinsic filtration. Highly unlikely that those filter below 100 microns, nominal.
So, the water will likely have to be diverted through 1 to 5 micron absolute filters.