it has scale insect , if you check under the leaves normally along the midrib of the leaf have a look for a brown scale and scratch off also on the stems , the poo if I can say that on here is very sugary and that what the mould is feeding on "sooty mould"..

You can either spray it with something that kills scale insect or what I have used on my orange very small bush is methylated spirits on each !!! leaf !!! as I didn't want to use sprays as it had a lot of fruit on it and wiped each leaf over but you have to keep checking for weeks as if you miss one or two it all starts again

Hope you find this useful

Gg

Thanks!
Will try your method as not keen on sprays, and I use some of the fresh leaves in cooking.