I take it you have tried removing it ? It is quite soft, but very messy. I saw a very good article on this somewhere, let me see if I can find it again.
I had it cover the back wall of a newish tank, but once I wiped it off it pretty much disappeared. It did grow in the filter to some degree, until I covered the filter with foil, to prevent sunlight getting in. Tanks are in a south window, so they get a lot of daylight. I have surprisingly little algae trouble though, for the most part. Let me go look for the article.
Edit. Can't find the one I'm looking for, I guess I did not book mark it. But you can use 200 mg
erythromycin phosphate/10 gallons water, to help kill off the bacteria.
Physically you have to remove as much as you possibly can. When you go to remove it, take all media out of the filter and replace with filter floss only. It will catch the bits that get away and then you can toss it out and return the media you had.
Ah, just thought, maybe you have a canister filter ? if so, If you can, turn it off, unplug it, whatever. If you have an old HOB
, use that to filter the bits out, if not, use a net to catch as much as you can before you turn the filter back on.
It can be related to overfeeding and lack of water changes, so it got worse while you were gone because of that, perhaps. You can't really get rid of it all, it's always there, waiting for favourable conditions. So be really careful not to overfeed, keep filters clean and do regular water changes. If the original cause is not solved it will keep coming back, unfortunately. I think mine was mainly due to the tank being new and having little to compete with it, and now I rarely see any.
It can be a huge pain but it can be managed.
Edit.. it is always introduced, with a plant or even with water from plants or fish, from other tanks. It really can't be totally avoided, you can only make the tank inhospitable for it.