Do NOT use the laundry soap!!! Never, ever! (If by "Mule laundry soap", you mean 20 Mule Team borax -- you won't hurt anything by using it, but you probably won't clean much, either.)
The brown line can be people goo. But if it was there when you opened the pool, there's a fair chance it's a type of liner deterioration that won't scrape or clean off. Basically, if the baking soda doesn't help at all, it's something you'll have to live with, until you replace the liner.
Sorry.
(PS, don't waste your money buying cleaners. But if you find you just can't help yourself, make SURE you check the labels, and that they specifically say "Safe for liners" or some such!)
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