The only way to know, my friend, is to turn the leaf and see if there are any insects. Inspect it with a magnifying glass or a microscipe for traces of eggs, shit 🤢 and insects which for the most part are not invisible but difficult to see with the naked eye. If you don't find any trace of it, don't worry there is nothing to fear. It could really be just water or an fucking caterpillar that has eaten the leaf. Find it and kill it 🤣