You will want to change his water more frequently if his tank is 2.5 litres. That comes out to roughly to half a US gallon. Despite the filter being present in that tank, it will not take care of the ammonia build up enough to reduce weekly changes.
In a 2.5 litre tank, I'd do 2 100% at least weekly to take care of ammonia build up (You can space the changes out so long as you are changing all of the water's equivalent twice a week). You may be told otherwise, but this si just my opinion for your tank size.
For diet, blood worms are not a good staple diet. They lack certain amounts of nutrients that a good pellet brand has. The can be effectively used as a meal replacement or a treat during the week however.
What temperature is your tank and is there any possibly way for you to post a picture of what the white spot looks like? Does it look like grains of salt sprinkled over him?