Am i correct here? #greenTaxes #priceMechanism #inefficientTax #DeadweightLoss
Yes you are correct, provided of course that plastic bags have no external effects. Then that triangle is the deadweight loss. 

However, green taxes are typically implemented for a reason and if plastic bags have an external effect the supply curve is not equal to the social marginal costs. The idea of the taxes is to bring the private marginal costs in line with the social marginal costs. If plastic bags do have an external effect than that triangle is not deadweight loss but part of a cost that society bears.