Less visible or invisible, we are not fish so we will never really know for sure. I am sure most people who use fluoro believe the fish cannot see it. Personally my applications are for chironomid fishing which I think makes a real difference; and low water steelhead, which I don't catch enough to say if its better than my 6 lb Ultra green.
You mention chironomid fishing - so I'll bite! I also use FC for chironomids because the diameter is smaller than mono, which will allow me to tie on very small flies if I need to. Also, chironomids don't move (much at all) which will allow trout to inspect my offering and casually sip it if it chooses to do so. I want every advantage I can get, so FC fits the bill for me. Otherwise, when I'm trolling searching patterns, I just use 6 lb. mono and it works fine. Trout will take searching patterns like leeches much differently than chironomids. They will typically commit and strike hard, so being "stealthy" isn't as much as an issue here.
On the river, I use FC for my coho roe fishing. Again, it's what works for me. I believe it's just one of many factors that increase my chances of catching coho, which I believe are the most difficult salmon/steelhead to entice. However, when drifting blades, I just use regular mono for the same reason I use it for searching patterns in the lakes.
In the end, go with what works for you!