It might well be safe, but the question is what are the downstream affects of reducing the population of mosquitos (or other bugs in the water that this substance impacts). Maybe they've exclusively positive, but I assure you determining that is no walk in the park.
I don't know, what are the possible effects of antibiotics? What are the possible effects of uprooting weeds, which some organisms may depend on? What are the possible harmful effects of putting out wildfires?
For every action that someone might think of as beneficial, there are all sorts of "harmful" effects that can be posited. In the examples above, these harms are not even just theoretical, and yet people do not constantly wring their hands worrying about wildfire fighting, weed control, or the proper use of antibiotics.
That this seems to happen only with mosquitoes seems quite weird to me, especially when it occurs even with efforts to reduce mosquitoes that spread malaria and other diseases. People are oddly very worried about some nebulous harm caused by reducing real and serious harm now.
Wildfire - controlled wildfires are actually beneficial to prevent the real bad ones and help refertilize. So they now let them burn down if possible and just control the edges where necessary.
Weed control - poisoned groundwater from herbicides/pesticides is a huge headache, so these are tightly regulated on substances and amounts.
Antibiotics - multiresistant bacteria have taken over since years. In turn, antibiotics are now prescribed more rarely and for longer periods (to make it resistant survivors less likely) and new antibiotics had to be developed.
"Mosquito dunks linked to declining bear population - farmers struggling with deer populations"