I've been recently talking a lot with my ex crush and he's said things like how he thinks I'm attractive and pretty and stuff, but then today he ends up going offline for me when I can tell he is online, as he had 'seen' my message. Why did he do this?

There's no telling for sure.

This happened to me a couple of times, and I spent some time over-analysing it. In the end, it could just be that he was in the middle of something (playing a video game, cooking lasagne, riding a bicycle??) - and he was able to quickly glance at his messenger, but didn't have sufficient digits free to reply back.

On the other hand, he is your ex, and maybe he still feels awkward about how things ended. Or he feels angry or bitter?

I'd wait a while, see if he gets back to you eventually. If he doesn't, then maybe he is ignoring you for some reason.

Then it'll be up to you whether you feel the need to find out why, or whether you leave him in the past and move on to a new adventure.