How to remove large areas of oxalis (yellow flower) in lawn before resodding or reseeding

My English oak is starting to get brown spots on the leaves. Can you please advise what you think is happening?

Many of my perennials and shrubs did not make it through the winter. I have found these very bright (almost fluorescent) stringy roots throughout my mulch and soil. They appear to be very invasive and I think it may be what has killed off many of my plants/flowers. I can not find anything like them on the internet. I believe it possibly came from bad mulch. Can you identify this? Harmful? How do I get rid of it?