Why are these balls when immersed in water completely invisible?

Same water, but these water marbles are invisible in the water, while ice is not, why?

What is in the basin above? There is water, of course. But not only that, but also in the pot contains many water marbles of all sizes, made up of a compound called sodium poliacrylate . This compound has a very high water absorption property, with the ability to absorb water about 200-300 times the mass itself.

But when placed in a water environment, due to the same refractive index, these balls become invisible.

These marbles, when held in hand, will refract light and distort the image, so we can easily see them. But when placed in the water environment, due to the same refractive index, these marbles will suddenly become "invisible" , as shown in the short clip below. Science is truly amazing, isn't it?