The picture worked this time, that's a neat tree
I had similar issues when I was misting my trees
. It is almost definately mineral deposits. It's not a huge issue, but it may be worth while to you to really give your tree
a good soaking and misting with purified water once a month (or more frequently if you can afford more purified water) to remove those deposits. The main concern will be that the deposits may eventually clog you soil, so if you soak it good you'll hopefully wash the deposits off. Make sure the tree
drains very well when you do this though, you don't want to drown it or have root rot.
As far as the fertilizer, I think you are ok, because if you are training the tree
properly you'll cut back any major growth that you don't like and in theory be left with a stronger, more robust bonsai, not neccesarily a bigger one, despite what the nursery says. Again, just to be safe make sure you keep the fertilizer light and not more than once a month or a little less often than that to see how the tree
I may suggest, if you have the ability, to get a bigger tray as well. The theory is that the water in the tray will evaporate and humidify the area just above the tray and the tree
will soak in the moisture. The tray you have it on now looks like it might be just big enough, for what that is worth. I've seen people use lunch trays to a nice effect. Also, make sure that your gravel isn't blocking drainage holes...
As far as pruning, if it's a tropical tree
it should be in a fairly stable, warmish environment, so pruning whenever shouldn't be a problem. The "No more than a third of the plant
at a time" method should also work out for you. But a lot of times a third of the plant
is a lot, so I don't do that much pruning most of the time. Also (again!
), keep in mind that after a pruning you'll want to give the plant
time to restabilize, so don't do drastic prunings often, though it doesn't usually hurt to go snag a large leaf or two when needed.
I hope this helps!