We get glitches with Township sometimes. One of them is in the store, showing you have something new when you have already looked and should stop showing something new on the store icon.
We get a similar problem with the events board occasionally. There was one that kept showing an exclamation mark above the events board. It didn't matter that we had viewed it a million times already
If it is definitely a decoration it is displaying new for you and you aren't missing something new in the other sections of the store. There isn't a lot you can do. It could right itself when you actually do get a new decoration or it could likely be fixed when the next update comes out.
If it doesn't resolve itself, you will need to use the in game support and put a ticket in with Playrix explaining your issue. Unfortunately we always get a Bot deal with our queries at first, and sometimes they give a useless answer and shut our open ticket on us! If this happens just persist, and open a new ticket and you will eventually get a reply from a human