A lot will depend on how the ransomware program is designed. Technically anything connected to an infected computer or device is potentially at risk. This includes mapped drives/Shares. There apparently is some ransomware that has gone after NAS drives. See the following link for one article that discusses the issue with Synology NAS devices.
https://redmondmag.com/articles/2014/08/07/ransomware-targeting-synology-nas-servers.aspx
The main thing to do is prevent the intrusion/malware from reaching the local network/computer in the first place. This includes good surfing habits, i.e., avoid surfing porn sites, opening spam emails, having good antivirus/security software that scans on a regular (or daily) basis, ensuring all computers and network devices are updated to the latest versions/firmware.
Password protecting a Share may not prevent an infection from an already infected PC that has that Share mapped. The better option is to lock down the My Cloud by setting a Share to Read Only access or No Access, turning off SSH and FTP and requiring a password to access the Dashboard. Obviously using strong passwords is also needed to prevent other methods of attack on the NAS.