the algae below the gravel isn't hurting anything. don't worry about it. or lower the gravel level so you can't see as much build up.
basic algae solutions:
reduce light - if you can't do curtains, is there a background on the tank? how long are the tank lights on every day?
reduce feeding (nutrients for algae)
increase maintenance/water changes to remove excess nutrients - how often do you do water changes and how much?
do you have live plants? what kinds? do you add any fertilizers or carbon supplements? if you have lighting for plants but algae is taking over then you need to give the plants a boost so they can outcompete the algae.
might as well get rid of the common pleco, sooner rather than later. it's not going to clean much algae (as you've discovered), it is going to poop a lot and contribute to waste buildup, and it gets far too large for your tank. any algae he does eat just gets converted to waste, which in turn feeds all that hard-to-reach algae more and more.
even though bristlenose plecos will eat some algae, they will eat what they want to eat. that may not be everything that you want them to take care of, and you will still have to feed them appropriate foods in addition to what they graze on.
between the bn pleco, SAE's, and shrimp you already have, there's no fish you can add that will help any more than the crew you already have.