What colour was the bug, did it look round, or thin, did it have visible legs and/or wings. From the sound of it it moved fast, is that right?
It's really hard to tell what it might be without pictures but it can be really hard to get those for bugs, so as detailed a description as possible will help.
My first thought for webbing in the medium is actually algae, but I could be wrong on that.
I've moved your post to the 'Pests/Diseases' section in the hope you will get more knowledgeable replies than me in this section