I wouldn't worry its probably just playful behaviour.
I don’t think it’s much of a problem. And actually Guppies do eat some algae as a snack, so the fish could have a little on him and he found itHi, i'm new here so please excuse any mistakes.
In my freshwater community tank, i have one(out of about 25) male guppy who appesrs to be "cleaning" other fish? He's done it with a large angel, and a large golden gourami so far(that I've seen). The Gourami was sitting still, and moving it's feelers out of the way, and tilting to.give the guppy better access etc, on both sides, then moved off to join the usual melee with no sign of any ailments and both are feeding well. I've not noticed any 'flicking' by any fish, and have a U.V-sterilizer in a sump, and use parazin once a month. (The aquarium itself is densley stocked, but the system has 50% extra water in sumps with enough filtration for double the stock level). I've never seen this behaviour in freshwater fish before (have had marine cleaner shrimps do it) and don't know wether to be concerned about the guppy harming the other fish, as it picked around the gills which the recipients did not like(elsewhere they seemed to like/tolerate it). Has anyone else had this?