Algae on your plants sucks up light which would otherwise get to the plants, plus it's unsightly. So, yeah, it's kinda bad for your plants.
Plecos don't, in my experience, do that great a job cleaning plants and decor. I still recommend a hungry molly any day for removing green algae from plants, especially the filamentous sort of algae.
Just remember when you get a million different pieces of advice on this issue that algae isn't magic. What works for one person works because of their aquarium's particular balance. It may or may not work for you. A healthy, balanced planted aquarium generally shouldn't have big algae problems so figure out where yours in out of balance.
So, you could try reducing your light period to eight or less hours a day. Many people swear by that. You might consider whether any products you put in the tank are boosting phosphate levels - algae love phosphate. Lots of PH stabilizing chemicals are phosphates - like "Neutral Regulator." Algae also love all the goodies in fertilizer and if you're adding more nutrients than your plants can use, that could help boost algae growth.
You might want to check how much you're feeding your fishies. Food + fish = ammonia. Note that these levels are miniscule in a cycled tank since the ammonia is absorbed as quickly as it is generated, I'm not talking about levels an API kit can measure.
When ammonia is generated in the tank, three things absorb it: Nitrifying bacteria (the biological filter), plants, and algae (yes, also a plant). Aquatic plants and algae need nitrogen, which they get preferentially from ammonia in the water. They suck it up in competition with each other and with the nitrifying bacteria. The plants tend to beat out the algae for nitrogen and other nutrients, leaving the algae unable to flourish.
If there's more ammonia than the plants want to suck up, the slack is taken by bacteria and algae. The bacteria make nitrate, and the algae grow if they have sufficient light and micronutrients.
So, mild overfeeding won't necessarily give you an ammonia spike BUT it can over fertilize your tank.
If your plant life is abundant and growing vigorously it will use virtually all available nitrogen and macro/micronutrients and algae should fall to the wayside. I have very little algae even though I use Neutral Regulator and probably over fertilize with the micronutrients and I leave the light on easily fourteen hours a day and definitely overfeed sometimes. I just got some tiger shrimp to hopefully eat up what little algae is hangin' on. Probably if I quit using Neutral Regulator and was more careful with the micronutrients, that algae would go away with help from the shrimp.
