as said above the slime coat is being eaten and can damage gills and scales if attached to stongly.
must not be enough algae in the tank for the otto to eat, i'd add algae waffers. and in spare tank or plastic tub grow some algae, by filling the tub with tank water and placing in some large pebbles, then place the tub in a suuny/warm location, once algae has grown place stone into tank, once it's clean add a new one and put the cleaned one back into tub.