Are those stones glued on to the surface? If so remove them. I would remove that moss too. Check the soil under those rocks and moss. It is most likely a peat based mix which is completely inappropriate for a bonsai. The mix is likely to be too wet or could be too dry (less likely). In either case, the plant will suffer and show similar symptoms. It is bit too late to do a full repot into a proper bonsai mix. June/July is more appropriate.

You can put the loose rocks back after inspection if you want.

If you can put the plant outside in a bright area with morning Sun at least. It was in a greenhouse so it is used to bright light anyway. Or if it is going to be indoors put it in the brightest spot possible.

Water it very well if the soil is dry and then wait till it is almost dry to water again. It is easier to judge if you use a "Tell". A bamboo chopstick (the plain kind) or a wooden dowel with sharp point is a good tell. Insert it to the bottom of pot and let it be there for minute and take it out. If it comes out dry and clean then it is time to water. Over time you would be able to tell by holding the pot and feeling its weight.

After watering hold the pot at a 45 degree angle and shake it up/down a few times and excess water inside will flow out of the bottom.