Nope, not Klong Plu, that’s the big waterfall on the west coast of Koh Chang where you pay 200 Baht to get in.
Klong Prao waterfall is in the same area but is much smaller and far less of an attraction which is probably why the plan, years ago, to make it into an attraction never took off. In the dim & distant past a road was built from Klong Plu waterfall and ran along the back of the valley to what was going to be a parking area and I presume visitor centre for Klong Prao waterfall. I remember taking a look years ago but there was no obvious path, from where the gravel road ended, to the waterfall.
I then forgot about it until recently
The road now can still be followed for most of the way, but the important final section is way too overgrown. You’d need to bring a machete with you if you wanted to walk it. However, it isn’t too difficult to bypass the road and go along the river. Just a matter of heading upstream from the spot where Baan Camp Chang bath their elephants and then taking the first tributary on the left. I find that following a river is a sure fire way to find a waterfall.
There’s a fair bit of clambering over rocks required but nothing too strenuous. There are several levels each with a small waterfall. Not really for swimming, there are a couple of far better spots in the main river to cool off in. From tracks next to the river it looks like there are quite a few wild pigs around here too.