Harmless protesters are safe in the USA. They don't need to be covered up. Covering them is effectively an acknowledgement that they are not in fact peaceful.
On the subject of that protester from one video being a murderer on another, not really. If you only have a single source this problem exists and is unavoidable, so much is clear. When you have multiple sources from different angles it becomes possible to correlate the person seen from the back in source A with the murderer in source B and with that identify that person in source A as the murderer.
In another photo, he is seen from the front, somewhat peacefully protesting. (his daughter was in the vicinity and held a weapon that he could have used)
If the face had been hidden in the protest photo, then that photo would not be useful for solving the murder.