My blenny is always doing stuff like that! Sometimes he will go up behind the powerhead and start munching. When he gets towards the front of the powerhead the current catches him and he gets flung off then comes back and he does it again.
Sometimes he will stop on a rock after he gets flung and look out of the tank, I think he is checking to see if anyone was looking or something haha.
Sounds normal though, seems lawnmower blennies have alot of personality.