Sometimes water looks blueish green because there are tiny plants called algae living in the water. These microscopic plants are bluish green in color and when there are millions of them in the water the whole lake or pond will turn that color. Other times the water looks that color because of how the light is hitting the water. If your family has ever been to Green Lakes State Park, the water in the lake is bright blue or green because of how the sunlight hits the water and reflects off of minerals in the water. Here is a link to the DEC page about the lake. https://www.dec.ny.gov/outdoor/70328.html