No, your eyes don't deceive you. Tropical fish live inside tropical slimes. They love to bounce around and possess an irrational fear of buckets

Tropical slimes are an aquatic variant of the slime, which is composed of water and can hold two tropical fish within themselves. Their eyes and mouth are dark blue and their body being brighter blue in color, their outer layer is a lighter blue in color.

Tropical slimes mostly have the same AI behavior as slimes. Tropical slimes hop around while being able to swim in water and climb ladders.

Unlike regular slimes, tropical slimes are passive toward the player at first. However, when a tropical slime is provoked, it becomes aggressive and attacks. This affects only slimes that are attacked, and does not trigger other slimes in the area to attack.

Even though a tropical slime is the size of a medium-sized slime, it does not split into smaller slimes when killed.

More info: https://minecraft.gamepedia.com/Minecraft_Earth:Tropical_Slime

Texture by https://www.curseforge.com/minecraft/texture-packs/rebearth-remastered