Sorry for the slightly less than friendly response, anguyen. Welcome to the forum; very glad you found us. There are a number of really expert bonsai folks around here; unfortunately I'm not one of them. Hopefully a few more will stop by.
You've already told us it's a juniper and the soil it's in sounds pretty organic, if it's holding moisture that long.
What I have learned from reading this forum a lot is that you did the right thing by putting it back outside-- assuming it is "back." Junipers don't do well wintering indoors. The trouble is you just got it and you don't know what conditions it's been growing in. What you wouldn't want to do would be move it from indoors (if it's been kept indoors before you got it) to very cold outdoor conditions without letting it harden off. But since your temps are so mild, it should be fine.
It sounds like you will want to repot it into some more free draining medium. I think now is not the ideal time to be repotting it, but someone who knows more than me, will have to help you balance out which is higher priority, getting it out of the too damp soil or waiting to a more ideal time to repot.
Pictures are always helpful... Here's the instructions on how to post pictures here.