It doesn't look like you left any room to plant anything. That's what will soften it up.

The rocks can be moved in order to add plants, but not sure what more I can do ?

some ideas:

It looks fine so far. It will look much better when your landscaping is in.

Where is this located?

One thing I would do is paint or stain the wall in back.

My feeling is that you need plants behind the waterfall as well as alongside the water so it at least has the feel of being part of a larger scene, even if it isn’t literally. So if you can move the waterfall out enough to fit plants, that might be best. Regardless I might think about putting mirrors on the wall and putting tall plants relatively densely right in front of the wall so it looks like it goes farther back than it actualy does.

The "core issue" is that you need to create a scene (as mentioned by NH Babs), which includes more than just small plants tucked into the fall. How much you disguise the fall is a matter of your sense of style.

After few hours of work, the fountain looks now like this...
Do u feel it’s better?
Any ideas?
I don’t see anything else to add a part from plants....?

Thank you for all your comments:
Few clarification :
- where do u suggest to put the mirror ?
- should I paint the wall the mirror differently ?
- the rocks will be moved and I will put plants on top and on the sides, just want to finish the work first then get plants
- this fountain is in the corner of my garden which is in middle of landscaping project :)
After few hours of work, the fountain looks like this now :) and I personally don’t see if I should I add more things but plants ?

I would put two relatively large mirrors, one on each of the two walls and meeting at the corner.

You could grow some Monstera deliciosa and let it climb the wall. Then it will create a more lush look. You can put in little hooks or something into the wall or even suspend some wood in some way for it to attach to. If you have cold winters you may have to keep it inside over winter.

Another option would be to attach some lattice on the wall and hang potted bromeliads, orchids, or whatever other lush plants you want. Monstera or even variegated/ philodendrons type plants could also climb wood lattice. I would use or make a thicker wood lattice in that case. Eventually the lattice would be covered with plants and you would only see plants no wall or lattice.

Bromeliads, some orchids, philodendron, monstera etc can all be grown in bright shade. I noticed that corner appears quite shady. They would enjoy humidity coming from the waterfall.

It would be easy to water them with a hose sprayer.

Some type of lighting would be good for night time enjoyment. If you want to put some seating out there so you can have dinner or drinks by the waterfall :) Maybe do uplighting near the walls and a pond light in the pool at the bottom.

Variegated plants/monstera or philodendrons that had some white mixed in with other colors would brighten up that spot.

This isnt exactly what I had in mind but the gist is there are vining plants, bromeliads, hanging plants behind the waterfall :)

https://www.google.com/search?q=diy+tropical+waterfall+with+hanging+plants&tbm=isch&ved=2ahUKEwix25PMh83gAhXHsFMKHQ33C-YQ2-cCegQIABAC&oq=diy+tropical+waterfall+with+hanging+plants&gs_l=mobile-gws-wiz-img.3...8922.10293..10776...0.0..0.95.362.4......0....1.........30i10.e8qqrelKKwY&ei=rrhuXLHMH8fhzgKN7q-wDg&bih=766&biw=412&client=ms-android-verizon&prmd=isvn#imgrc=z1sbw7H2C39vEM