A toothbrush will not solve this. Change your watering strategy.
I do hope this plant was only indoors for the picture? It should be ouside. With your soil, frequent summer rain will flush the calcium from the soil. If it is really dry, you can help this by every month adding a spoon of vinigar to a liter of water, and water the plant with this, water till the water runs from the bottom, then slowly keep watering till the can is empty.
The next time you water, you should add fertilizer to your water.