Because kelp tends to be more on the organic side, I would not recommend it for hydroponics. Organics can be used in hydro, but more often than not people report the same issue as you, funky affects in their res. Add some beneficial microbes, and if you don't have any, do a res change. Beneficial microbes like Hydroguard and Great White can break down the organics and convert them to nutrients instead of just rotting in your root zone and attracting bad microbes.