It seems like there’s some disagreement whether or not freely-floating helium balloons in the sky are good/bad for the environment. Some say they harm both land- and water-based animals. Others disagree.
I’m inclined to think balloons can be harmful. Come on, helium balloons are clearly not made by nature. Some animals might think it’s food once it lands back on the ground. Who knows what damage it can do?
Better to err in the side of caution. If you’re attending a party or hosting one and you’re having helium balloons, don’t let them fly away. Dispose of them properly.