New Lawn Yellowing Question

First the details: Zone 7 (Long Island, NY), shade/sun grass seed (bluegrass, rye, fescue mix) sand/clay soil mix evenly graded with gentle slope Lesco starter fertilizer at germination (12-24-18)
I planted this lawn four weeks ago and watered daily up until three days ago. I have a sprinkler system and was putting about 1/2" of water on the lawn each day. I have cut it once and will cut again today. New mower with very sharp blade. Lawn is deep green and surprisingly full already.
The problem is a small area, about the size of a surfboard, that is yellowing out. Also have some appearance of mushrooms in various parts of lawn in the mornings. The weather has been in the mid seventies to mid eighties with no rain for the past three weeks. We get heavy dew in the morning which can take until noon to dry off when overcast. Also has been humid about half the time.
I'm afraid I overwatered after germinating the seed and now have some type of fungus turning the surfboard spot yellow. Please let me know if this makes sense, and if possible how to remedy.
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