Hey Lin ~ sorry to hear about your loss.
How long have you had the little pleco? From reading a lot of people's experiences, it does seem as though many plecos prove themselves to be extremely tough and live forever, while another one can barely handle even small issues. I'm also wondering if you acclimated the little guy to the new tank? Even if every test result you have indicates that both tanks share the exact same parameters, we don't test for everything so perhaps that was a problem if, in fact, you did not acclimate it.
As you allude to, your pleco isn't going to thrive upon, or even eat every type of
algae and so they often starve even though there is an obvious abundance of
algae. In fact, as many plecos start reaching adulthood, quite a few of them start to rely less on
algae and become scavengers who live off of uneaten food and other
detritus...which could explain why your larger pleco is still going strong. Point being, you could be right about the starvation.