Weird things happening over last week

If you are using iCloud, adding a Safari bookmark or adding / deleting Contacts (for example) will cause that change on all the Apple devices using that Apple ID.

Those changes shouldn't be unexpected though, and being constantly harassed for your iCloud password is not normal. If you are concerned about your Apple ID security, use Two-factor authentication for Apple ID - Apple Support. It will effectively slam the door on any unauthorized use of your Apple ID credentials.

Your user name implies you live in Saint Helena; I just don't know which Saint Helena. Two-factor authentication isn't available everywhere on Earth: Availability of two-factor authentication for Apple ID ...

Here are some additional suggestions: Security and your Apple ID - Apple Support

Do you, or does someone you know, own any Apple devices other than that MacBook Pro?

The reason for asking is that the symptoms you describe imply someone else may be using your Apple ID, because it can be used to synchronize that information among those devices.

If it's someone in your family it may be an innocuous manifestation of someone sharing your Apple ID. That just leads to confusion because everyone should have his or her own unique Apple ID.

I understand your frustration. It's hard to know what you're seeing, but your startup disk ought not to look like a silver little disk.

With Apple ID concerns like yours, it's best to contact Apple directly, for a number of reasons. First of all your Apple ID is considered confidential information that should not be revealed on a public website like this one, and solving the problem will be difficult unless you can discuss those personal concerns. Second, if there is in fact a problem with your Apple ID, only Apple can fix it.

Also, Apple has the ability—with your consent—to see what you're seeing on your Mac, as you are speaking with them. They do not have the ability to control it or to otherwise use it under any circumstances, and you must never let anyone do that.

To get started click here: Contact Support, and click through the options that address an Apple ID concern. They will lead you to the same Apple Support documents I already provided, so keep looking until you find one that says "My problem isn't listed" or one that will otherwise lead to a way for you to contact them.

Set aside an hour or so of your time and they will help fix what's wrong. You may have to pay them, but their fee is only $29.

Do not perform an Internet search for "Apple support". Use the contact link above.