I follow pretty well the suggested trzining advice in Xert. (even a little bit harder and longer). I’m in the build phase (endurance). But my FTP is going down. ± 1W a week.
Is this normal?
Perfectly normal unless you periodically ride a BT style workout every couple weeks or so to “reset” your signature. That might be a BT workout from the library, a RAMP test, a group ride with hard segments and a sprint, or a Zwift race.
Keep in mind suggested workouts in Xert adjust automatically to whatever you are accumulating.
You can mix things up whenever you feel like it.
You may also consider switching to “No Decay” method if you don’t like testing too much, or if you’re in a base phase of training. This is my progression over the past few weeks, and you can notice how my TP is tracking with my training load, despite no actual Breakthroughs.
That’s not magic 
You’re a magician !!! What did you do? What did I wrong?
Read above 
Please some more information. My FTP is now 15W higher. How can I avoid it in the future?
If you don’t use it, you lose it. There may have been some other adjustments, but if your decay is changed to none, your TP will not slip if you have a steady TL, without BT, during for example base training, or off season.
OK, is something with ‘no decay’.
When I put now my target watt (in the training advisor) on my Garmin I get a TTE of - (minus) 66:50. And now??
When you change decay method, it only applied to all future activities. To apply it to older activities, you must find the first activity where you want the new decay to be applied and then flag/unflag it, which will reanalyze all activities after it (until the next locked activity) using the new decay method. I’ve changed it for you for now. If/when you get into outdoor riding where you can push for more regular BT’s, you might consider switching back to Optimal Decay.
Re: TTE… Contact support. Include details of data field settings in Garmin Connect Mobile, images of the Garmin data fields, etc…anything else that will help us troubleshoot it. Cheers