For the tank size that you have you need to do water changes about every 6 days.
When i chance water i rinse everything in the tank with warm water. But i don't like the green algae on my stones so if i see it i just rub the stones under the water to take it off. With time it also has some kind of slime lair so i do rub it out.
I do the same for the plants. You don't have to take the silk plants out. It ok to have live and silk plants in the tank. Just make sure he has enough space to swim, and it not too overcrowd . Bamboo check the roots, make sure he will not stock in it. Not sure if you can trim the roots. Moss ball i never had one but someone on this forum wrote just rinse it every time when you change the water. Also next time when you buy any live plants always rinse it and put it for quarantining for about 2 wk in the water. Because live plants can carry ich and if you quarantine it how i understand it would die without host.
Also for the tank size that you you really don't even have to have the filter at all. You can just do full water changes every 5 days which you have to do anyway with the sponge filter. Or you can do 1-50 and 1-100% water changes a week.
If you do full water change make sure you acclimate the betta to the new water so you don't shock him with the temp difference. If you needs tips on that let us know.
Now if you still want to have sponge filter i want to give you the link where you can find helpful information about it
CHECK OUT the post #22