Meaning of XX below Slat FLap Trim on A350 with flap setting 1
The number below the flap indication is not the position of the flaps/slats, but the position of the flap lever. When the lever is moved, the number becomes cyan indicating that the flaps/slats are in transit and when they have reached the selected position the number becomes green.

Edit: As per FCOM, there is no mention of amber XX indication for flap lever position at all. Checked in a simulator that total computer failure will not cause indication to turn XX.