There are several problems visibly from your photo of your Imperial mandarin.
Imperials tend to get bushy and crowded so when pruning its necessary to remove complete branches--and don't clip around the tree as that only makes it get even more bushy.
The result is a tree with a clearly visible trunk as well as the main branches.
If the trunk and branches are exposed to hot winds or sun then gardeners whitewash the trunk with lime or a flat water based paint as seen below.
The foliage on your tree is pale green and is likely affected by a deficiency of zinc or the toxicity of excess boron and possibly nitrogen.
The foliage below is typical of a happy and healthy plant.
The other issue is a little curling of leaves that could be due to dryness or salt or boron toxicity.
