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.

Thanks.