If you still haven't found the secret area, look closely at the right side of the waterfall, there should be a faint glowing circle signifying an entrance.
Turn on your sound. The glass panels signify a mirror, so the two sides north and south are supposedly a mirror image of each other with some differences. For example, one square has some "grass" while the other side doesn't. Go to that grass square and press confirm. You should hear a "ding" sound or something similar, signifying you've found one. Find them all then talk to the cat and that should be it.