We have the same algae that is in your pic . It is a type of FA just different from that in your pic with the LMB. It did not kill your LMB. It is possible that you had a DO crash but not because of small amounts of live FA. The blob type algae is indicative of nutrient rich water and shows up about this time in our ponds.

FA floats up on O2 that it expels through respiration. That is how it gets from the bottom to the surface.