NORTHEAST SCOTLAND
I got a this bonsai tree about a month ago and it says its a Sageretia. I have looked at care guides online but there are some conflicting advice. I have been keeping it on a west facing windowsill and used fertiliser twice (chempak bonsai)
When i first got it i notice some a lot of the leaves had black bits on them and yellow patches and they were falling off (images provided) but didn't think much of it considering it was at the beginning of autumn and perhaps it had been under watered.

Then after a while i noticed white fluff on the soil around the base of the tree after waiting a few days it had spread over most of the topsoil. I looked around on the internet and thought i was probably over watering and not giving it enough air flow so i followed some advice from someone else's question and scraped off the top soil that had the fluff on it.

It was all good but now the fuzz has come back a bit around the tree base and half of the leaves (i think the older ones) are like i said earlier. I don't know if it is relevant but soil on base of the trunk that has the main bit of fluff is very hard compared to the rest of the soil.

I was considering re-potting it but thought to ask here incase its a harmful fungus etc. Also to see if the way i am currently caring for it is suitable.
I have included some images to help.

Welcome. I am not sure what the problem is, the soil looks poor. There are also some dead bits, the fluff is probably a mould from being wet. Not a problem if watered correctly. So, water when almost dry do not water to a schedule. I would leave re potting for now, get the watering right and keep it in as much light as possible. The difficult time of year is coming up, if it is still alive in spring I would re pot then. This species is notoriously difficult to
keep alive, you would be much better getting a hardy species so that it can live outside. Buy a book, join a club if you are serious about bonsai. Don't believe all you see on the internet.
Geoff.