activist shareholder; outspoken shareholder
A shareholder who actively voices opinions or demands changes in a company's management, often at shareholder meetings.
An activist shareholder asked tough questions to the management at the shareholders' meeting.
Recently, the influence of activist shareholders has been growing.
株主 is a general term for 'shareholder', while 物言う株主 specifically refers to an activist or outspoken shareholder.
Compound of 物言う (ものいう, 'to speak out, to object') + 株主 (かぶぬし, 'shareholder').