It is the result of the high molecular weight components which are formed due to the influence of calcium and bicarbonate ions at the liquid water interface. The scum can be removed in two ways
By filtering the calcium ions,
By adding acids to covert bicarbonate ions to CO2.
Very little scum is formed on a cup of very strong tea. As the acidic tea polyphenols themselves partly neutralise the bicarbonate ions. It also should be noted that less than one mg of scum is formed in a cup of tea and it is not known to be harmful to human health.