Is this bad, what can i do, how do i stop happening again?

No yellow leaves, looks very healthy

I got it 4 weeks ago so i very new to this...worried...love this bonsai very much!!! Thank you so much for help

The green on the trunk looks like algea, that may be a result of you keeping it too wet. it can be carefully taken off with an old toothbrush and some water.

how often do you water the plant?

You don't get algae from keeping trees too wet. They die off sooner because of overwatering before they would get algae.
These are tipycal mass produced mallsais from asia, they keep them in an extremely humid enviroment, so they grow fast. Once they reach a pencil thick "trunk", they cut it back let it grow some leaf, then voilá, its sold as bonsai.

you won't get algea on a dry trunk... as algea do not grow under dry circumstances.

You don't seem to understand. Of course algae don't grow in completely dry environment. But simply watering a tree too much is not enough for algae to thrive. It needs a very high humidity level. So unless you live in the middle of a tropical rainforest, you won't get algea from too much watering. What you get is a dead tree due to overwatering.