What is....?

To me it looks like damage caused by the leaf rubbing against your frame when the fan is blowing on the plant, and nothing to be concerned over. Is it only 1 leaf or is it everywhere? If it is everywhere, then that is a different issue. After looking at your diary, it doesn't appear to be anywhere else. You may be expecting too much of your plant; no plant will grow 100% perfect leaves 100% of the time. Your plant is a living organic being and not a robotic mass produced perfect machine! Two other observations from looking at your diary - 1) I hope your ties are elastic or have the ability to stretch. Tying knots around your branches with inelastic or rigid tie material (like wire/pipe cleaners/string) can strangle the branch as it grows, as branches not only get longer, but thicker too. Far better to tie a loop around your branches, starting with an anchor point, over the branch and then back to the anchor point, to then tie off with a knot there. - and 2) in my recommendation, try only removing old, yellow leaves. Those big, green, healthy leaves are producing the vital energy needed by your plant to grow her biggest and best flowers. Those leaves also act as a "storage" site for carbs, sugars, amino acids etc that your plant will draw upon during flowering. This is why the older leaves will go yellow during flowering, even though nutrition and health are ideal. It is far more efficient for your plant to re-use those elements than to create them from new during a high energy activity such as flowering. Those "bud sites" that people are worried about as being shaded, are relying on those big, green, healthy leaves to produce the energy that the flowers need to grow. Fewer healthy, green leaves = less energy produced = less stored reserves of vital grow elements = slower and smaller growth. It is that simple. By removing healthy green leaves, people are forcing the plant to grow new healthy green leaves to increase energy production levels again and to re-manufacture carbs, sugars, amino acids etc that the plant had cleverly stored in those big, healthy, green leaves, all over again. This takes a lot of energy, energy that could otherwise be going into flower growth. After growing cannabis for 35+ years, I am yet to see even one plant that gets rid of its own healthy green leaves for no apparent reason. Cannabis has been evolving for 100s of thousands of years to become the peak of perfection that it is, Her main/only goal is to breed. This requires the best flower growth possible. It does not involve getting rid of energy producing, vital element storing, healthy green leaves. Sorry, but I feel better now that I have had my daily rant about the pointlessness of de-leafing. I think I better go and have a lie down. Again, sorry for ranting. I hope this helps you, or someone else reading this, in some way..... Organoman.