I used Chemi Clean many times and have since they invented the stuff. I doubt it harms anything. If it did my reef would not be three decades old. Is it natural? I have no Idea. Are Polyfilters, carbon, ASW and Kalk natural? I doubt it. Chemi Clean is a tool like a powerhead is a tool. My reef gets some cyano, not a lot but it gets it. It does not bother me much but I am sure that in all those years there is a lot of stuff in there that is just not going to be siphoned out. Is my reef overcrouded? yes it is. Stuff has been growing in there all of this time, do I overfeed? yes.
Why? because I have some animals like a moorish Idol which I want to feed three or four times a day as an experiment. Why?
because this is a hobby and I can.
I also feed my fish bananas and avacados, why? because they live between 12 to 18 years old and spawn all over the place.
If your reef is old and healthy enough you will get cyano and probably hair algae. It is not a disease and will not harm anything directly. Chemi Clean helps eliminate it as will removing all of your rocks, cleaning under them and changing the water.
Not likely going to happen in an old very healthy reef tank.
There are plenty of pictures in my gallery, see if it looks like the Chemi Clean hurt anything.
Have a great day.
Paul
ps. I do not work for Chemi Clean