Weird spots on seedlings.

Reused from a previous cannabis grow? Or reused in the sense it was from the garden and you grew some plants in it last year? All soils should be refortified with amendments end of season and allowed to rest, some people use a crop called green manure where some plants are grown as their roots go deep and bring up nutrients then they are plowed under and left to decompose and enrich the soils. But this is done on a larger scale for fields and such not so much home gardens in small plots. Give em some feed and if it was cannabis soil give them enzymes to breakdown the old roots. This will set em back if its not done, google something called crop rotation for gardens. Good Luck!

Was the soil well rinsed? What's the pH of the soil? The spots themselves just look like water spots from watering but those would be my first concerns with any issues in recycled soil.