I recently started a cloud trial of Confluence to see if it is viable for our team, and somehow ended up on 2 cloud instances:
I don't get any response from admins on Activeniu so there's no way to remove my account on that server as I am not an admin there. Is this something that Atlassian team can fix?
Welcome to the Atlassian Community!
You are assuming accounts work in a certain (traditional) way, but things have changed a little bit.
The older way to handle accounts is that a system has a list of people, with unique identifiers for them, and a human would log in to that system with that account.
We have moved on from that model in most places. We have accounts that are provided by identity provision services that are independent of the systems you log into and can be used in many places.
If "unmask" and "activeniu" are Atlassian Cloud systems, then they are using the second model. You don't actually have an account with them, you have a 3rd party account which has been granted access to those systems (an Atlassian Account probably, the same account you are using to post here in the Community)
There is no "remove account" from an Atlassian Cloud service - there are no accounts nin those services, just a record that "this Atlassian account can use it". The closest you can get to "delete the account from the Cloud system" is "have an admin remove the access my Atlassian account has"
You can, of course, delete your Atlassian account. But that removes everything, not just your access to one system. If I deleted the Atlassian account I use the most, I would no longer have access to the 40-ish Cloud systems I help support, own, or raise bugs in, and 50k posts here in the community would suddenly become unattributed.
Anyway, the shorter version of this is: You don't actually need to do anything here. Just don't log into the activeniu system. After a while, the admins there might realise you're costing them money and hence disable your account, or, if they've lost all interest, the site will be shut down through inactivity.
TLDR: You don't need to worry about deleting your account. Just ignore the systems you don't care about, you'll be fine.