Hello! I just got my first bonsai (a Serissa) and thought back to a time I visited a garden that had a bonsai exhibition. For each tree, there was a small white sack that appeared to have some mix of soil, moss, compost or.. something (?) inside of it. The sacks were placed at the base of the tree trunks. I am curious as to what that might have been. Does anyone have an idea?

Hello and welcome!
Easy one.

Organic fertilizer inside tea bags.

Thank you! I've done a bit of reading and it says the bonsai should be fertilized once a month (while not in winter). Do you find this to be true? How does that method compare to the fertilizer sack remaining on the tree?

Fertilizing is something of a science for some. And for others (me) a matter of just doing it. My trees get maybe once every 2 weeks liquid fertilizer. Next to that, I add fertilizer pellets on the pot which slowly break down.

I think having only liquid fertilizer once a month and nothing else will give too much of a peak in availability. The little sacks will over the period of weeks continuously give of fertilizer, leading to arguably a smoother growth.

Hi all,
last year in spring I had switched from liquid fertiliser to Biogold pellets for the same reason - however they developed mould and small flies all around the trees.
Clicio had suggested me the use of teabags but the pellets had already broken down by that time, so I removed most of them.
This year I've forgotten to buy teabags so I used pellets again, dipping them gently under the surface (akadama) - do I have to worry and remove them, should they develop mould and flies again? Or can I just leave them all until they decay completely?
Thanks!

Ricky, the mould is perfectly normal, organic fertilizers have to be broken by bacterial activity so it's part of the process.
The little black flies are called fungus gnats and are a sure sign the soil is keeping wet more than it should.
Yellow sticks and Neem oil on the soil keeps them in control (and less watering, of course).

Thanks Clicio.
I will be more careful in watering. All my trees are currently in 100% Akadama so I should not risk overwatering due to damp soil, as I normally water when the soil is dry (easy to tell with akadama).