Usually algae is a sign of imbalance in the water, either nutrients or too much light. I’d try lowering the light or getting some more fast growing plants that’ll help shade the Monte Carlo tree, the other option if the hair algae is only on the tree, let it go free. I’ve seen some tanks that people cultivate the algae. I have a moss ball that reproduces and I have mini moss chunks collecting and growing and kinda like the look so far.
The betta will probably always have the hunting instincts towards shrimp and tetras. You could try cleanup crew snails like ramshorn or bladder, maybe Pygmy Corys if you have a plan for them if betta decides he doesn’t like them either
UphorbiaUphoria 
Usually algae is a sign of imbalance in the water, either nutrients or too much light. I’d try lowering the light or getting some more fast growing plants that’ll help shade the Monte Carlo tree, the other option if the hair algae is only on the tree, let it go free. I’ve seen some tanks that people cultivate the algae. I have a moss ball that reproduces and I have mini moss chunks collecting and growing and kinda like the look so far.
The betta will probably always have the hunting instincts towards shrimp and tetras. You could try cleanup crew snails like ramshorn or bladder, maybe Pygmy Corys if you have a plan for them if betta decides he doesn’t like them either