[FREE][AlienWare]Ethernal

... and then how often does decay occur?

Surprisingly enough aliens are NOT radioactive. What you and others are referring to was the change from the former levels to the new tier system. The underlying metric "lifetime ARP gained" wasn't changed though.

I'm also not getting any credit for watching twitch streams, how does that part work?

They are aware of the issue and working on it. Seems the problem originates from a change in twitch code. After that AWA twitch tracker stopped to register your time on twitch and grant ARP accordingly.

Right now it works for some while for others it's still bugged.

Edit: There's an unofficial fix by jk you could try.
"The easiest fix to try is to change your Twitch language, since the language is part of the URL used to load the extension and a different URL means your browser won't remember that the file "doesn't exist" and might load it properly. You can also try clearing your cache.

If those fixes don't work, you can open your browser console (Ctrl+Shift+J on Chrome-based browsers, Ctrl+Shift+K on Firefox) and paste in this code I wrote that force loads the part of the Twitch extension that tells Alienware Arena servers that you're watching Twitch: https://gist.github.com/jkmartindale/c2a2cb83e6a2e889c7e3aa7b37b5fedc"

Usually ARP (levels are based on ARP) decreased because i.e. AWA stuff delete user's created content which was i.e. upvoted a lot (usually happens if user's content was duplicate of existing content) or user just spam with new discussions (you get ARP for creating discussions). And, as far as I know, these are common ways of decreasing ARP at the moment.
A year ago, time then comments also had vote option, if discussion was deleted (for what ever reason) all comments in that discussion were deleted too and votes for those comments were gone too, so sometimes it was pretty big ARP decrease if discussion was long.