This will not be a clean and tidy answer: whatever is formed on the tealeaves that is not dissolvable immediately and that stick to the tiny air bubbles in the water when the water hits the leaves.
Most abundantly would be tea dusts. There would also be residues of fungal matters that fed on the tealeaves as the leaves were laid flat throughout the fermentation process. The fungi are killed during firing but the residues remain. That is why this foam is more easily found in fermented teas such as oolongs, blacks, and puers.
It is okay not to do anything about it, but some people prefer to scrape the surface of the infusion mass to get rid of them.
