Bettas do change coloring over time. Especially after bringing them home and getting them healthy/happy. They will/can change while they age, keep an eye on the white streaks. Look for missing/tearing of the fin or if it starts to be white AND transparent. This could mean trouble. If you notice it's just changing color and not falling off or becoming transparent, he's probably just healthy and happy!
As for the algae, any time you have light you're going to get algae. It will usually show up on decorations as they don't move and always get light on them. Over feeding gives algae food, so does over lighting. Try and keep your grow light on for 8hrs only, if you want to watch him more than 8hrs/day add a small LED light or something that won't feed the algae. No matter what you do it hardly will ever go 100% away. Spot treat your decorations by removing the algae when you do water changes. It's unsightly but it really won't hurt your fish. If there's algae everywhere, there might be an underlying problem that needs to be addressed. But if it's just on decorations/high light areas, that happens.
beautiful fish!