Abalone not looking good

[Subnautica]

Today I noticed my Abalone looks like this. He does move around slowly, but he doesn't look like he's firmly attached to the glass like before. What's happening?

 

[Tahoe61]

They need a decent amount of algae present. Mine have been nocturnal, so out and about during the day it may be searching for food.

How old is the tank?
Are you adding any substitutes for algae?