Gunk in window corners - ok to remove?

I've started working on cleaning up the windows on our trailer.  Just like everything else on this trailer they are in seriously rough shape.  There is some sort of silver substance in some, but not all corners.  I don't think it is original because it is very poorly done, but I wanted to get others opinions on whether it should be ok to remove.  I guess I'm mostly concerned that the PO tried to fix loose window panes and by removing I will create a frame that isn't able to hold the glass tightly.  
This is my first time trying to post photos, so hoping it works!
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