This is the new message you see for any unsigned app. This does not in any way automatically mean the app you're trying to install is malicious.
While software from any vendor can be dangerous to install, code signed or not, the OS makes the assumption an unsigned app leans more towards the possibility of being bad than good.
ria36's response should be all you need to do to install it.
FWIW, I use the same software for my Smart Label Printer 650. About the only thing I would stress is that you make sure to get it directly from Seiko's download page. Trust it from nowhere else.
It's the second one on the list, which Seiko VERY confusingly started over with on the version numbering. The 64 bit software is currently at version 1.3, and the OLDER 32 bit only software is version 1.8.2.