I cant really see in your pictures what your plec is doing, but i know what you mean, our plec has done this plenty of times, not to mention our other fish do it quite a bit too, they like to suck onto the side and get blown around in the current, quite often our little oto's ride a blade of grass in the current, its SO cute,Im sure there is nothing to worry about, infact plecs are meant to like strong currents in the tank, so hes probably really enjoyinng himself, while your sitting there worrying about his strange behaviour hehe.