I don't have experience with mead/melomel. But it looks like you have too little headspace? For beer I would leave 1/3 of the vessel for headspace so the foam doesn't go through the water lock. I think the seperation is normal

The process is normal - fermentation creates carbon dioxide that rises up as bubbles once it reaches a concentration high enough to no longer dissolve in the liquid around it. Bubbles then stick to whatever they find around that can act as a nucleation site - in your case, the bits of pear - and transport the thing to the top.

The concern here is that once you get very high krausen, you'll potentially block off your airlock with bits of fruit and cause a small geyser as the pressure builds up and your cork pops off. Do try and monitor it to make sure that does not happen. You could replace the airlock with a blow-off tube that has a bigger diameter - just seal one end to the bottle (you'd normally use a rubber bung of adequate diameter and run the tubing through it) and put the other end in a jar of water. This way, you also have an airlock but any misbehaving bits of fruit can just end up in the jar via tube.