Hi Minh,
You have eyes of an eagle...
Those are slow release fertilizer like Osmocote or etc. I've been using them with no ill effect to the neps. Other fertilizer like fish emulsion and seaweed extract can be use if diluted, like what you have done.
From my personal experience, if you apply the fertilizer on the media, it will eventually get mushy, slimy and smelly caused by the growth of algae or blue green algae. If you spray it, you'll less likely to encounter the same problem.
According to some, the fertilizer can be more effective if apply to the root zone compare to foliar application. I guess you can experiment on that...
Good luck and thanks for sharing your results.