The exclamation point is specific to Lemmy. In Mastodon, search for and follow the user main@rblind.com or @main@rblind.com. You should now be able to follow and interact with it from Mastodon.
That said, Lemmy does not work well in Mastodon. @main@rblind.com acts as a bot user that reposts (boosts) all posts to the community in Lemmy. But it also reposts all top level comments. It will make your Mastodon timeline an unordered feed of comments.
I’ve complained about this, but apparently it’s a common pattern for community based activitypub networks (still unclear why). Activitypub doesn’t yet support communities properly and Lemmy has no plans to change this pattern.