Not sure what the white on his fins could mean - I hope it's just his coloring changing, and not a problem. Someone else on here could probablly give you more info - s' really pretty, though :'P Shark has two little white tips on his ventral fins (Think I got that right? *Googles* Yes, I did).
Bubble nests are literally just that - a nest made out of bubbles. I'm not expert on the matter, but from what I understand the males build then when they are happy/horny/to impress females to encourage them to mate with them. (Don't quote me on the next part) The male, from what I understand, collects the eggs and fry and tucks them up into the bubbles?
Either way, from what I hear it's generally a good sign if your Betta male is building one, but not neccesarily a bad sign if he isn't. It's different for every Betta.
Here are some pictures of them(not mine, googled them):
Shark's been working on his on/off since I first spotted it this morning (which for me was almost noon XD)
If I got any of that info wrong, let me know!