Lawnmower Blenny
Usually they're real hardy little buggars , don't think I ever saw a sick one. But yes they need a mature enough tank to have sufficient algae growth to graze on. But you may feed it dried algae ( nori) in qt or ask to buy a piece of live rock that has algae on it for qt.
Most of what's on the rock will die off but it's basically as if you had bought ' cured' rock.
Also if you buy it from lfs and it doesn't sell right away you could visit the fish a few weeks before getting it ... fish that have been in the store more than a month usually don't die....That way lfs sort of does the qt for you!
Of course where I used to work, made the orders, reception, acclimation and qt and would tell good clients if the fish they wanted had been in a tank where there had been a death and to come back in 3 weeks to a month for it.
But now that I am JUST a customer ... I know that some lfs are rotten and I only shop where there are truly passionate knowledgeable people who think like I do.