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It is probably moss, and I think you shouldn't seed for about 6 weeks (due to the weedkiller!). A bit late to give you this advice but ideally you should have treated it 6 weeks ago, as April is a great time to sow grass seed. It gets a bit too hot for good germination by the beginning of June.
Rake out the dead moss, but leave the seed sowing for a few weeks. If the patches are small in area you might try sowing after three weeks, watering well. If there is no sign of germination after 10 days then sow again. You'll just have wasted a few handfuls of seed. Once the seed has germinated you should hand weed for a couple of months as grass seedlings are sensitive to lawn weedkiller. idally you should not treat a newly sown lawn with weedkiller for six months.