Whenever your fuge has light over it the algaes within are photosynthesizing. This means they are taking in CO2
and releasing oxygen as a bi-product. Basically that process can interfere with your ph by either raising or lowering it depending on the context. That is why many people have their fuge lights kick on just after their main tank lights go out, in hopes of minimizing or even eliminating ph swings.
I personally run my fuge lights 24/7 and have found that this works with very little shifting of ph. I have a pinpoint ph monitor on my aquarium and during the day my ph is about 8.28 and at night drops to about 8.12 and this does not seem to have an adverse effect on the fish. As for caulpera going sexual I am not sure what he means by that. I would recommend, lighting wise, either a reverse lighting schedule or a 27/7 arrangement.
I almost forgot to mention dissolved oxygen levels as well. You can imagine that dissolved oxygen levels are much higher when your fuge is working away crunching in all that light and releasing 02. The oxygen levels drop when your fuge has no light since the algaes are not releasing as much 02.