Autoflower: light in colour & slowed growth

Hey guys,
Newbie here with auto's. I have completed a number of grows but this is my first time with auto flowers. I have two seeds, one is doing great and one has slowed its growth over the last 7-10 days. It is candy cane from crop king seeds. They are in 5 gallon buckets, regular mixed potted soil, minimal nutrients ("grow big" by fox farms) used about 1/8th strength one feeding. Feeding ph'ed water about every 5-7 days as needed. 600 watt light about 18 inches away. These two are about 4-5 weeks old. Temps are a steady 19-21 Celsius (70ish), hunidity 45-55%. My guess would be possibly over feeding? Stems are still bent upwards to the light but the leaves itself are light in color. I will attach a pic of my healthy auto to compare as well. Thanks for any help or info!
(I realize my pics are under HPS lighting I can upload better pics if need be).