It is not really a security breach since one of the main purposes of a NAS is to share files between different devices on the LAN.
Each account on the Goflex Home has its own Backup and Personal folders. These are password protected. If someone can access these folders without your password then that is a real issue.
There is also a Public Folder and as the name suggests every account can access that folder, but they still need their own account password to log in to the NAS.
The 3rd type of access, which doesn't require any account credentials (apart from authorization to access your LAN/wifi) is UpnP/dlna. This is a media streaming service, similar to Windows Media Player which broadcasts media files to everyone on the network. You can chose what shares you want to stream in Seagate Share > Shares ; right click on a folder or file and an there's an option to share with media devices .I think the Public folder is shared by default, but you can turn it off.
It sounds like your housemate has accessed the media files that you have allowed to broadcast on your network. It is worth checking your settings since any photos or videos that you save in a folder that is shared with media devices will be visible, but only to people on your network (unless you haven't secured your wifi!) and can't be deleted or modified, only viewed.
Dlna is very useful as it means that you can play music or watch movies or view photos on various devices in your home, TV's, tablets, phones without a lot of security getting in the way, but you do need to be choosy about what you make available.
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For the sake of your own health and sanity, if your data is important to you, keep a second copy on another storage (a backup)
