After a brief SLAM upon opening and tons of pollen, I was needing to backwash frequently. I didn't do a deep clean last year, as we only had the pool up 2 1/2 months last season because we upgraded.
I'm guessing the fibrous stuff I found was the pollen. I had it in the pool and vacuumed some up a couple of times. (It was spongy and not slimy when it would clump together on the floor of the pool. I could get some of it with my leaf net.) I had quite a bit I scooped out of the filter from the top of the sand with my hand. If you are unsure about doing a deep clean, don't be afraid, it was quite easy. Just don't be aggressive with the hose! Here are a few pics:
Before we put the hose in:
First bit with the hose: (yuck!)
I could tell the difference in the water force coming out of the return immediately. I back washed and rinsed, expected a little gunk to come out of the return when I first turned it on, but it was nice and clean.
Here's a link to the deep clean tutorial:
Deep Cleaning a Sand Filter