That is hair/beard algae, nothing eats that crap. usually means you light it on way to long. Just shut it off. Yes you have to feed the plecos. And something i think is really important for plecos is the pH. Yours is to high. Remember 99% of plecos you can buy are wild caught. That means they come from low pH. 10 gallons are not good tanks for plecos also, most of them need there own teritory. Also you nitrAtes are high. that means you havent done your water changes right. Start doing them more often. please take the advice you daughter will have fish for many years in that tank, or you will give up completely. I do understand.