Betta Fish: Does he look healthy?? Also what is this fuzzy stuff growing on my Mopani wood???

Hi! This is my first ever post on reddit! I've been a lurker since I got my first betta back in December.
I started working at my local big corporate pet store (horrendous idea I hate it here so much but the discounts help me budget for now)
And I kept feeling really bad for all of the fish and just our animals in general because even my stupid managers tell customers incorrect information AND I'm not even allowed to deny a betta sale because everything in the store is apparently up to "insert store name's standard" INFURIATING, but enough yapping, sorry:
I haven't had the best luck with Bettas because of the misinformation at my store that i wasnt aware of until i started losing fish (I've been through three including this guy) But I'm trying so hard to learn! My first guy, Leviathan passed because I didn't understand water parameters and cycling my tank properly and it was a bit of an impulse buy, the second guy, Hades, I don't know how he died… my water parameters were PERFECT and the tank had been cycling for two months and he wasnt showing any signs of any of the illnesses ive seen (ick, pineconing, velvet, ect.) So i assume he may have just been an old guy… 🙁

This is Belial! He's my third and final betta and I am so anxious about keeping him healthy and safe. My water parameters are still perfect other than my PH being a little high… I've read somewhere that you're not supposed to chase the PH of your tank and that you can't use the PH stabilizer if you have live plants so I've been trying really hard to add tannins into the water!! He seems really healthy, super energetic and a big appetite so I'm sure I have nothing to worry about I'm just anxious and just want some reassurance 😭😭

And that stuff on the Mopani wood!! What is it?? It looks really gross like fungus or something! Is it dangerous? It's only on the lighter part of the wood and only on one piece (I have two small pieces)

I also added a picture of the whole tank!
He's in a 10 gallon with 2 snails and 3 albino corydoras (I've heard that Cory's are supposed to be in groups of 6 but they seem happy and healthy and they honestly don't even really school together from what I've seen) I have the corys because I wasn't having any luck with shrimp at all they kept dying off :/
His water temperature stays between 75°-82°
I know he doesn't have many hides but he doesn't seem to mind honestly he never seems stressed. He DOES however like to squeeze in between the wall and one of his decors (the one that says "dead fish tell no tails") and I had to move it away from the wall because he kept getting stuck ):/

Sorry for such a long first post!!! I care a lot about this guy and I'm working with what I have so please be gentle with me if I'm doing something horribly wrong!! Any and all information, tips, or comments are so SO appreciated!! Thank you if you read all of this!!