NFT stands for non-fungible token. Art is the popular example at the moment--one piece of art is not equal to another... they're different. But NFTs can be used for more than just art. Any individual piece of data (songs, health records, event tickets, etc.) that you want to create on a blockchain could be an NFT.
Fungible tokens, on the other hand, are typically currencies. Real-world money is fungible--my dollar is worth the same as yours regardless of condition. Bitcoin, Ether, Koin, etc. are all fungible.
But what about Koin's mana? Mana isn't represented by either an NFT or a fungible token. It's a property of Koin. It can't exist outside of that context. So, when you spend mana, your Koin is now "empty" and needs to "refill". That doesn't impact the fungibility of Koin, though, because Koin cannot be traded/spent/sent unless its mana is full.
So, by spending mana, you're not changing the fungibility of Koin, you're sacrificing liquidity for a period of time.
Even then, you are still going to want to spend your mana because once it's full, it stops recharging. It's a free resource as long as you keep using it.
P.S. What does "trustless" mean to you? Hit reply and let me know. Thanks to a reader discussion, I'm brewing up a series of posts on the topic of trust in blockchains and specific examples on Koinos.