Lactic acid bacteria infection.
Honestly, if it is just a gallon batch or something I’d probably dump it rather than go thru the trouble of fighting to save it.
The infection developed due to poor sanitation.
If you really want to try and save it, whip the crap out of the cider with an eggbeater while you add a strong dose of sulfites (k-meta, campden etc). Ideally then you need to use a wine filter on it ... but if you don't have a filter, you could otherwise just keep it under refrigeration permanently and drink promptly. Some use bentonite after this process to remove some of the bacteria but it does not completely do the trick like filtering. If you go the bentonite route read up on how to do that properly ... bentonite has a long settling period to clear.
Also, as it is often Lactobacillus in cider that causes ropiness (not just Pediococcus or Leuconostoc as in other wines) this is specifically a high pH infection ... so do something to lower the pH ... if you have any of the acids used in winemaking such as malic, tartaric or citric you can go ahead and *carefully* add just enough to add a touch of acidity to the cider (or if you are using pH test tape, shoot for pH 3.5 or less).
The whipping not only incorporates the sulfites and re-incorporates some of the ropiness, but lactic acid bacteria like CO2 and degassing that way helps get rid of excess CO2.
If this is the standard Lactic Acid Bacteria infection causing ropiness ... then for what it's worth, the slimyness is from a change in the carbohydrate/sugar molecules and it is still drinkable (to the degree the infection has not made it nasty tasting). But if left untreated it will get worse and you’ll end up likely dumping it.
Like I say, if it’s a small batch I’d dump it and start again ... but *first* reading up heavily on proper “sanitation” (keeping things clean and sanitized) in winemaking and cider-making.
EDIT: ok ... I re-read your original post. If IF the slimyness is only across the top surface as you said ... then it may or may not in fact be a LAB infection ... in any regard; siphon off the wine with a tube into another container making sure that you stop well before you suck into the siphon tube any of the gunk at the surface. Then, as above add the sulfites (campden, k-meta etc), adjust pH and refrigerate. No need to whip. If it keeps going south you'll know soon enough and the infection will continue and the wine will get worse.
On the other hand, if the slimyness is actually throughout the rest of the cider then my original post (above) stands.
