What are the slimy bubbles in soaked kombu?

I have made dashi a couple of times now, starting soaking kombu in water. And always I found these little bubbles on the leaves, containing white slime:

I have never worried about them much and simply cut them away when eating the kombu later, but what are those bubbles? My hypotheses are: 1. just random bags of alginate slime, or 2. natural growth due to parasites or other incorporated matter.