I keep seeing the sentence "Your word strength has decayed by not practicing" The verb is to practise so this should be practising - unless it is different in American. In English it is " I practise" but "I have done lots of practice".
By the way, we do the same with license and licence. I do know that it is used differently in America as you have licenses where we have licences.
What can I do that this message disappears. Can I click on the Button "Practice all Skills" or Do I have to click on every single Tree Element?
From what I understand, you can do it either way. One other way to do it would be to click on vocabulary at the top of the screen, and choose the option to practice your weakest words.
I've only been at this for a week and a half, but I think a mixture of reviewing parts of the tree and using the other options is a good way to revise. If you don't like timed practices, you can choose to do the practices without the clock (but you get more coins with the timed practices!)