It isn't UFFI. UFFI is very soft, turns to powder when you touch it, very slightly yellow color, and when you see it in this sort of location it's squeeze out from above. My WAG...... It's a pile of hard ass efflorescence that someone's been trying to paint, cover over, or otherwise deal with moisture in all the wrong ways. I'm watching a job now where, what we thought was very thick white paint is actually very thick efflorescence, and it looks kinda like this.