Unrecognized charge of Nebula App on Apple Account

First see if you can recognize the charge:

How to see what you were charged for

1. To see your purchase history, go to reportaproblem.apple.com.
2. Sign in with your Apple Account.
3. A list of purchases appears. To see family members' purchases, select the Apple Account button and choose a family member.
4. To see the full receipt including tax, tap or click the date. Multiple purchases, including subscriptions, might be grouped onto one charge.

If the charge is for a subscription that you no longer want, you can cancel the subscription. Find out where else you can view your Purchase History.

If you checked reportaproblem.apple.com and your emailed receipts and you're still not sure why you were charged, contact Apple Support.

You were most likely directed to their website by a link in a Social Media site or website for a horoscope reading. This started a subscription with that company directly and is a subscription that is not being managed by Apple. Yo will need to cancel the subscription with that company just like any other third party subscription. Here is a link that may be helpful to contact them and get that subscription cancelled.

https://24hours.support-nebula.com/hc/en-us/articles/28900802305297-How-to-cancel-subscription

It is likely that the person who directed you to their website received money to provide that link and gets a kickback from the users that they direct. You will find this frequently on Social Media sites. It is no longer a place where you can just connect with old friends and stay in touch with family, it has now evolved into fake accounts and fake reviews where users are only interested in making money off of others.