Aquarium plants that are covered in  thick crusty algae, green slime, or long hair algae is unattractive and very frustrating to get rid of. One treatment that has been going around the internet and used by people as a last resort is a diluted bleach dip. The formula is 1 part bleach to 19 parts water. 2 to 3 minute soak, rinse thoroughly.

Paul Krombholz, who first came up with the treatment gives us some pointers for using this treatment without killing the plants in the process:
 

Remember that the purpose of the bleach treatment is to get rid of hair
algae forever.  Once you have got your tank and aquarium plants free of hair algae,
the only route of re-infection is through new plants that you failed to
treat or by way of new fish or snails or other critters that are dumped in
the tank along with water bringing fragments of hair algae.  Snails can
also have hair algae attached to their backs.

The idea way to handle bleach treated aquarium plants is to put them, immediately
after rinsing, into a tank with the following conditions:

(1) bright light
(2) no hair algae in the tank (otherwise all your efforts are for nothing!)
(3) Some established plants already growing (Ceratopteris is especially good.)
(4) Somewhat nitrogen deficient but plenty of Ca Mg, K, and S.  (I don't
know why, but Rotala and probably some other plants do not get started well
under nitrogen excess.  Also, green water and cyanobacteria are not likely
to be a problem.)
(5) CO2 added (Leaf area may be destroyed by the bleach, but stems,
rhizomes, survive.  New plant growth has to come from food reserves or from
photosynthesis in stems and rhizomes.  In any event, more CO2 helps the
damaged plant recover.)
(6) Chelated iron in the water.  (The recovering plant has to grow new
roots, and may, initially become iron deficient.)

Any bleach not rinsed away will be only a trace, and will be removed as it
reacts with organic matter in the new tank.

Ambulia species  ought to withstand 3 minutes in the bleach.  They should,
like all plants be rinsed thoroughly immediately after the treatment. I've
even had Java moss survive 2 minutes.  The method I use is to mix up about
a liter of 5% bleach, put this in a plastic bag, set a timer for the time
(usually 3 to 4 minutes), dump the plants in the bag and hit the timer and
start sloshing the plants and bleach in the bag.  When the timer starts
beeping, I drain the bag, fill with tap water, drain, fill, drain fill,
etc. for about 5 times, and then put the plants in the aquarium where they
are going to grow.

There are not very many aquarium plants that can't stand 3 minutes.  Najas, Java
moss and Ceratophyllum are the only ones I can think of presently.