i'd wager you got a little fertilizer on the leaf and it resulted in what you see.
important thing is it stays isolated. if it doesn't expand it;s more certainly a drip like i suspect or just some early imperfection. could be an early leaf blemish which are far more common and often don't show later, so not much concern. those 3-bladed leaves are 'early' leaves.,, even the 5-bladed seem more prone to blemishes.