Superjohnny- One of my SAEs mysteriously died one day to, I don't know either. When my SAE's get that big they devour whatever food I am feeding to the rest of the fish as well as eating algae. Were yours eating tother food? If you want another SAE the size of a finger, I've got two that I would sell to you for the cost of shipping

NOw, the brown stuff on your leaves is probably not an algae. Maybe a bacteria or something. Is it slimy and does it wipe off easy? If so, I am pretty sure it is not algae. If it is a brown algae, I don't know why the Ottos don't eat it, I was under the understanding that it was a prefered algae to them.
With the algae tabs, what has worked for me is feeding the pigs first, I usually feed the general swimmers brine shrimp, BBS or something they really like first so they are full. THen, I add the algae wafers and break them up and put them into areas where my shrimp and ancistrus hang out. IME, most fish do not prefer the wafers, they will just eat them becasue they are there and they are hungry. That being said, I have seen my ottos eat the algae wafers, but not very often and only after they break down a little. Their bellies are usually full of algae, and I have little to no visible algae unless you examine a plant leaf under a magnifying glass. So, if you have any visible algae, they are probably fine. Also, IME they prefer not to eat algae off the gravel. They seem to prefer the leaf edges, driftwood, and sometimes the glass in my tanks. I think they are amusing little fish, that are excellent workers.
The only problems I have seen with Ottos has to do with water chemistry IME. The people I have seen that have trouble keeping them have harder water, higher Ph, and a higher nutreint loads. They definitely seem to prefer more pristine water conditions. Thats why they always look so bad at places like Petsmart.