Welcome to reef central =)
If you want opinions you really should be specific about basically the whole system.. What type of lights, flow, tank size, filtration, husbandry etc, basically a quick run down of the tank, maintenance and equipment list will rule out a bunch of things along with getting you opinions faster than every body having to ask these things(and they will =) .It's basically impossible to give an educated opinion in your case without the basics of the tank, and pictures of the things in question and the tank are a big help also.
There are a lot of things that could be going on from to much light/flow to not enough, alk spike, changing salt brands, changing lights, old bulbs /new bulbs, brown jelly, pests I can go on and on but it will all be guesses without more info and or pictures .
If I have any random issues first thing I always do is add carbon and do a larger than normal water change.. Having poly filter is a great way to rule out contaminants as it will turn different colors for different things.. Those two things are exactly what I have on hand at all times just in case..