have the same problem. I don't think anyone can help you on here considering i made a post about it and none of the advice really worked. i don't think your substrate has anything to do with it because i'm using dirt capped with floramax and i'm having the same problem. I used to have 2 t5's on my tank but i took out one bulbs today so i only have 1 t5 54w as lighting, well see how things goes from here. Try messing with your lighting, dim it, shorten the photoperiod. Anyways heres my post if you want to check it out.
I and other's attempted to help.
You seem to be of a mind that fertilizer's cause algae while those who run higher light CO2 injected tank's dump fertilizer's in every other day and they have no algae or very little.
Plant's need food,CO2.
If lighting drives the demand for food,CO2 past what is available ,then algae which need's much less of evrything will thrive.
If one cannot increase the amount of CO2 that light causes demand for,and one can't/won't feed the plant's sufficient food,then plant's struggle and algae thrives.
Cannot place too much light over low tech ,NON CO2,non fertilized tank without incurring algae,lot's of folk's try.(I have )
In higher lighting, high tech tank,CO2 and food for plant's are in high demand, Same thing can/does still happen. Folk's run uber lighting and cannot get flow,distribution dialed in, or cannot increase CO2 for fear of gassing fish but plant's are demanding more of everything due to lighting which drives everything.
They too could reduce lighting and thereby reduce demand from plant's but is hard to move folk's off their lighting which may cost big$$$$ so they spot treat,use chemical media,reduce fertz, dip their plant's,scrub hardscapes,perform water changes,blackout's,etc.(problem persist's/return's).
Decreasing that which drives the demand (light) work's well in both low tech.high tech if one has a little patience,but folk's want result's right now ,in much the same way they want fish med's to work right now.
Fish/plant's seldom get sick overnight,and cure does not happen overnight.