The larger the Fish, the more he/she needs to eat. To answer your question, no, you're not overfeeding. However, you could actually be underfeeding! Common Plecostomus are very large Fish, needing extremely large tanks (100+ gallons and at least 4 ft long) and plenty of variety in their diet. The older they get, the more animal protein they need, and they'll sometimes resort to sucking off the slime coating of a Fish, which will make them become susceptible to disease and they will eventually die from it.
Most of a Plecostomus' diet should consist on algae wafers, live algae, blanched vegetables, and driftwood, however, they will also feed on sinking pellets, live aquarium plants, frozen foods, gel foods, fresh fruit, cooked/raw fish, and ditritus, among other foods. It is best to allow them to feed on algae and driftwood freely every day, but also supplement them with algae wafers every other day, fresh blanched veggies every few days, and whatever else you think he/she will enjoy :)