I think it is algae now....

[waterbear]

It's not mustard. Mustard doesn't settle in the cracks. Also, when you brush or vacuum it it's back in place in about an hour or two. I suspect it's just normal junk from not vacuuming, possibly some pollen mixed in to give it color.
I would strongly suggest getting a better test kit that uses FAS-DPD chlorine testing such as a Taylor K-2006 or a TF100 from TF Test Kits. (google them, it's owned by someone who is a member here but I don't think he is active on this particular forum much but he is busy on another pool forum.)