That badge means the app starts, fails, and is being restarted over and over. It is not stuck downloading - something is wrong with the app itself.
The two common causes:
Not enough memory. The app needs more than your plan allows, or more than is left after your other apps. Stop or remove another app and try again, or move to a larger plan.
It needs configuration. Some apps will not start until they are given a database, a key, or a settings file. The app's own documentation says which.
What to do:
1. Remove the app and install it again - a failed first start sometimes leaves it in a bad state.
2. Check the memory figure on the app's card against what your plan gives.
3. If neither helps, open a support ticket and say which app it is. We can read its log, which you cannot see from here.