Can someone explain/help on why this is here and how to fix this, it’s in the trunk and after a wash I opened the trunk and found that, there was more build up of soap foam

 

Perfectly normal. The crud is debris that falls down the rear windshield and into the drainage "channel" of the trunk. As water drains, it carries the debris along the drain path where it accumulates under that plastic trim piece. Just clear the crud away and water will drain better under that plastic trim piece (see pic). Otherwise, you have nothing to worry about.

 

I just cleaned my buddies G out. His was clogged so bad that water would collect and not drain. Pressure washer FTW. The plastic panels can simply be pulled/popped off.

 

I thought I read somewhere- here on the forum, perhaps- that someone had trimmed the bottom lip of the drip channel trim panels to allow for better drainage?
This larger opening would help to prevent the "damming" effect.

Edit: found that thread here.

I just pop the trim pieces off, clear the crud, and pop back on.