Hi there,
Might have some things you can try. With your waterfall particle - find the material you are using for it - open that material up - set it to unlit mode then drag the base colour into emissive - this will make it glow without need for lights.
If this doesn’t work - set the waterfall to movable and the light and see if this helps, but ideally you want to try and make the particle glow.
Let me know how you got on.