My technique for getting fish to try veggies is to make small slits in it & put in small pieces of wafer in them. They can graze on the wafer/tablet food & get a little taste of veggies. It doesn't usually take long for them to just eat veggies alone.
Many plecos are omnivores & even algae wafers often don't really have much in the way of plant material. They usually have quite long digestive tracts. Is that real wood I see on the left side? If not, you might add a piece for him to graze on. It provides both wood fiber & "auf wuchs", a biofilm of micro animal & plant (algae) foods. Not all plecos "need" wood but many enjoy it.
But I'm not sure that's the answer to the stringy clear poo either. It can be a sign of internal parasites. Long poo is just normal for big plecs, clear is not.
