The chemical used is most likely Fluorescein which is a synthetic organic compound and has been in use for industrial applications for many years. It is a completely inert substance and dissipates in the environment (think large amounts of food coloring). In fact, one of the common names for it is D&C Yellow #7.
In this case this is coming from a storm and rainwater treatment system. This is part of the bi-annual inspections required by the EPA. One if the things they look for is where runoff actually goes to make sure there are not pollutants in those locations. That way if anything had been spilled on-site they know where the runoff would go. They then test water and soil samples wherever they see the green to ensure full compliance with the vast array of rules, laws and regulations.
Dye tracing like this is often used to track water flow for anything from glacial ice melt to the outflow of runoff from chemical plants. Something similar flowed from a Dallas hospital last year causing an uproar, but it was just routine testing to check for plumbing leaks.
Redditor FiMack, who claims to work as an environmental health officer, says they've used the compound for a similar purpose.
Fluorescein was originally the dye used to color the Chicago River during St. Patrick's Day before it was phased out after complaints by environmentalists. It's also used by optometrists to check the eye for scratches.
A video posted to YouTube shows a similar effect on a river near Victoria, British Columbia.