Plecos usually shy away from eating algae as they get older because they do not like it as much as they do when they are young, hence the need for a regular veggie diet. I feed mine spinach and zuchini every once in a while.
GEL, I prepare my spinach by putting it in the freezer while I boil a pot of water. When the water is boiling, I dip the spinach in the water for about 10 seconds or so, rinse it off, and put it in my tank. For the zuchini, I just put it in boiling water for 10 seconds, rinse off, and put it in the tank.
HTH