Test your water.
What size tank ?
How often do you do water changes and how much water do you change ?
Plecos need at least 30g or more. They often need driftwood as well as supplemental food. They don't live on Algae alone.
Algae Eaters including snails only eat certain kinds of Algae. You need to offer them food as well.
Don't add more fish till you know what happened.
To combat algae;
Lights on timers. Reduce to 6 hours daily.
Water changes weekly
Don't overfeed
Don't overstock
A well balanced tank won't have a lot of algae.