Algae on top soil

light and constantly wet = algea growth. as canopy expands, this will die off... as you stop using a clear cup and growth on side will grow off. it will steal a very minor amount of nutes. at this point it won't cause any issues. so, sometimes this happens early on becauase we tend to water a seedling a bit early instead of letting top 1" dry as you would later on in a bigger pot / bigger plant. if this occurs on 2nd irrigation, that's am istake. the roots are deep enough to wait for a proper wet-dry cycle. this extended period of moist top soil is an opportunity for algea. wait longer to irrigate. you wanter entire substrate (10% runoff if soilless) and you wait for the top 1" to dry (top layer changes color for coco). simple as that. frequency and concentration of fertilization depends on how much you are giving per dose.. with soilless you fertilize every single time with 10% runoff. with soil, it may be 1 of several viable options for frequency/concentration, but over time it'll end up being a similar mass per week or per monht realtive to size of plant and current growth rate.