How do I permit remote access to my WD Mycould to a certain folder without permission to all folders?
Have you read the My Cloud Dashboard embedded Help and or the My Cloud User Manual (https://support.wdc.com/product.aspx?ID=904)? Both have sections that detail how to setup Users and Permissions on Shares.
Currently the single bay/single drive My Cloud units do not support restricting access beyond the Share level in the My Cloud Dashboard. This means that all subfolders within a Share will have the same permission levels set through the My Cloud Dashboard Share page.
WD has decided that one cannot set the main default Public Share to Private. One can use the forum search feature (magnifying glass icon upper right) to find past discussions on various methods one can use to try and set the main Public Share to Private (or remove it entirely).
Use mycloud.com, see example image below and read the information provided.