lol my Aristotle does that, he lies upside down in an old urn, i have to look very carefully to check his mouth is actually moving, i swear he starts off eating the algae, then goes to sleep & drops off staying like it. he's a bristle nosed Ancistrus, i've had him either 26 or 27 yrs this month, he was just about 2cm when i got him, he's now about 16cm, he usually disappears when i try & measure him
i did inherit 2 plecs though, a friend had to get rid of her late husbands tank, so i had half of the fish & a work colleague had the other half, my tank was too small for them really (a tall, deeper, 4ft) as they grew to be 18 & 19inches with heads that were 4inches wide. I'm not sure how old they were but they were smaller than that when i was given them.
nobody else wanted them, not even the aquatic store. i had them about 12yrs i think. i had one of the smaller 8 inch ones as well for another tank, i was also given him when a friends boyfriend moved in & had to get rid of the tank, i had him 16yrs & he was second hand when he got him.
if you count the spines on their central (large on back) dorsal fin you can tell if its the smaller or larger variety. i cant remember if it was 12 & 13 or 18 & 19 spines, i'll try & dig out some photos tomorrow. but the one spine makes the difference between specific breeds & their sizes.
pretty plec of yours, i have been caught out many times with how still they can be!