If you can see it, just keep an eye on it. If the edges start to look worn or more smooth, it may be softer than the tank and wearing away. That could mean you are getting tiny pieces of plastic in your water.
If you don't drink from the tank, the only worry would be the effect this could have on your pump. As to that, your guess is as good as mine.
Worst case, you cut a hole in the upper portion of the tank, fish it out and patch the hole. Maybe put a threaded access port in the hole for the next time something like this happens -:S
