We can't really answer unless you give the parameters of your tank. We need to know Ammonia, Nitrite, Nitrate, Salinity, Phosphate, Tank setup, age of tank. The more information you supply us with the better we will be able to help you.
That brown polyp you have looks like a Palythoa. They are similar to zoanthids and are pretty common. Here is a pic I found of 1 type although they have huge differences in how they look.
Palythoas are very closely related to zoanthids, they wont bother each other. It isn't uncommon for zoanthids to only half open for a little while. If they stay that way for a while it might be a good idea to try moving them to a higher point in the tank for more light. Also, what lights do you have over your tank?
The angel didn't belong in a 20 gallon tank anyway, but it certainly didn't die for "no reason". Something wasn't/is not right. Could have been poor collection practices, parasites, acclimation issues...etc.
As for the brown polyp, see if you can get another picture of it. It may be a majano anemone, and not a paly at all. If that's the case, it will be stinging whatever coral it's touching.