Japanese Hornbeam leaves

Hi,
I'm a bonsai novice. I have several Japanese hornbeam trees that started shooting beautiful new leaves this spring. Unfortunately, after two week or so the leaves on some of the trees started to become dark green and shrunk around the edges. We still get some frost at night so I keep my trees indoors in a pretty dark place (unfortunately, I live in an apartment, and don't have too much room to keep them elsewhere).
Another thing is that it's relatively dump there, and in one of the pots I noticed some white mold on the soil, and on the bottom of one of the trunks.

Any advice would be greatly appreciated!
Thanks!

Get 'em out. A little frost won't hurt them. If they're leafing out they need light.

Thanks a lot! That's what I'll do. Hopefully they will bounce back:)

A word of caution. Since your trees are leafed out, you can't just shove them outdoors. Put them out in a very sheltered location at first. Gradually put them out into almost full sun. Try to keep them above freezing, or you may lose the present crop of leaves. They will grow new ones. In the future, put the trees out before they leaf out. Keep them outdoors in the fall until temperatures go below 25.
Iris

Thank you! This is most helpful!