What's the 'followupd' in notifications?
This is part of the private CoreFollowup System Framework at

/System/Library/PrivateFrameworks/CoreFollowUp.framework/Versions/A/Support/followupd


There is no documentation about it, but the header files found at (that's for iOS, but I guess the framework is the same):

https://github.com/nst/iOS-Runtime-Headers/tree/master/PrivateFrameworks/CoreFollowUp.framework

don't show any evil things.

I suppose, it's for "following up" on notifications posted by other applications. There is an internal database of which the statements can be grabbed from the binary. It only contains title, body, some IDs and dates. Example:

INSERT INTO notifications (item_id, title, body, unlock_label, relevance_date, activate_action_id, dismiss_action_id, clear_action_id) VALUES (?, ?, ?, ?, ?, ?, ?, ?)


So, no – you should and cannot remove it.

[EDIT]

Regarding the icon. Some examples of system applications having that icon: