Can someone diagnose this please??

I have 4 plants in my grow room and one off them has had this on The leaves for around a week now,Can anyone give me a clue to what it is??


I'm growing in jacks magic soil.

I'm also using A+B by canna they are getting 5ml off each into 2 litres off water, My other plants are doing fine as wine it's just this one that I'm having trouble with, I clean my grow room every 3-4 days for pests and things and I haven't found any yet ( Touchwood)

Thanks in advance folks!