IMO it is always a preferred option to manually remove unwanted plants for just the reason esshup mentions. Often it is not practical due to the effort required. It can be next to impossible to remove plankton. Often but not always the plankton you see on top and wind driven like you pictured is soon to be dead. It can't hurt to scoop it out and put it on the bank. I am talking about phyto/zooplankton not thick blue/green algae (cyanobacteria). I would scoop that out if possible without a lot of effort.